If you want to base this on the real world...
I had a friend who was 7'2". Now when you are that tall your height actually presents a health risk, though it connects as well to your weight (those who have played GURPS will have heard of the square cube law). Anyways one day he was very distraught because he had grown 2"- as an adult. Turns out the reason was because he had been losing weight. A human's height also varies by 1/2 inch over the day as the spine compresses or decompresses at night.
Thats why
A +3 size increase is considered roughly 10 times bigger. (ArM pg 18) So if I did the algebra in my head right that means a +1 size increase about x2.15 in volume on average. So the square cube law being what it is that's a little less than a 30% increase in height. So for an average 5'6" 13th century person where to get a +1 size increase your talking growth of about 1'6". To be fair that's to get from the middle of size +1 you could say the shorter end of that size catagory is 6'6". So a growth of 1 foot is probably enouph if you started from an average average size.
All that was assuming the Squarecube Law was rigorously adhered to. That all volumetric changes where because of an equal change over all three dimensions. People don't change size that way. Just as the example of your freind illustrated. He got taller because he lost weight. Technically from a mass point of view he shrunk. I imagine that if you had measured his volume before and after his weight lose that measurement would also show he shrunk.
You could then say that about doubling your weight accounts for +1 size increase. So the average for each size would fall around 0 is 120-160lbs, +1 240-320lbs, +2 480-640lbs. The obese flaw bears that out but but I'm not sure we want CrCo spells that turn us into weebles. (though honestly those I might allow 
) Sure it's "natural" in the real world to go from 150lb to a burly 300lb bruiser (search Christian Bale, Machinist, & Batman Begins) but then we are back to naturally increasing Str and Sta and a whole host of other things. Because it would actually be harder to change size without effecting those other game characteristics.
And on that point does anyone want to argue that human mass doesn't change naturally? I know my adult weight has fluctuated between 190 and 270...
Does it change without altering your Str, Sta, or Qik?
Does it have to? The question is can you alter size permanently, not whether it will alter other aspects of the character.
I think it does matter. My point still is that the sort of change being talked about in the spell is the not what can happen naturally to an adult human.
The only thing that can account for significant weight gain in a grown human is packing on fat and/or muscle. It's not packing on fat because there is no talk of the obese flaw. It's not throwing on 10 stone of muscle cause that would be a huge strength boost. It doesn't even seem like a weird blobby combination of both being added just right to balance just right. Because I just don't think there is a way to add a 120lb+ medly of muscle and fat to the same frame without altering characteristics.
No what the spell suggest to me is a growth of nearly a foot or more. Totally altering the skeleton reshaping the subject into someone double the mass but who's frame accounts naturally for their stats.
When my friend went from 7'2" to 7'4" his stamina and strength improved. Nobody is saying that going from size0 to size +1 would not impact your statistics, it could well improve your strength and/or decrease your quickness, as per the guidelines in ROP:M. And it seems what you are really taking issue with is not the principle but the scale.
He shrunk. As you explained it even though he got taller it was because he lost weight. He was taller yet smaller.
No one mentioned it as part of the effect. Also I shudder at the level that would be a fair for a Base Effect that adjusts size +1, str +2, and Qik -1 all at one and permanently.
Yeah that was pretty much my point of saying. (Bolds added.)
The size attribute represents an average increase of overall size of more then double with each +1. That's the scale of change the spell proposes. That's an unnatural change to me. Which is why in principle I'm more comfortable with Muto remaining the Technique for altering size.
Creo Corpus can't change size only when mature or grow one enfant to his adult size...
I can accept that with Creo Aquam and Terram, but normally you create new matter, other thing are elementals that can feed or make that part of them.
Hypothetically, what if it were a spell cast during childhood or puberty?
Then you'd have a very large child.
There is a diff Creo effect (defined in CrAn (and CrHe), if not CrCo) that rapidly matures a living creature.
Right, but that is not what we are talking about- we are talking about a spell that would make someone larger, not more mature. The argument against is that people growing to a larger size as adults is not natural. Of course, at level 70 you can permanently animate a dead body, which is decidedly not natural either...
I don't know. The vast majority of human bodies I see on a daily basis are fairly animated.
And have that vacant look, right? 
Agh, horror! I really had hoped that common fallacy (that to create something with unnatural properties you need Muto as a requisite) did not make it into canon books. What will be next? That to create something ugly you need Perdo, or that to create something intelligent you need Intellego as a requisite?
Frustration!
I'd already say that to create something with one of its natural properties destroyed would need a Perdo requisite (e.g. creating armour that has no weight, or fire that sheds no heat), although not to create something ugly. Althoug some of those cases would probably actually be Perdo with a Creo requisite, as destroying something's natural The intelligence would be a Mentem requisite, though. There are loads of examples in the corebook of things requiring a Rego requisite to be controlled after creation.
Why do you think it's a fallacy? If anything, I'd be dubious about the implication that you can permanently have something unnatural by creating it from nothing. There are plenty of examples of Creo guidelines creating something in an unnatural place/shape without needing a requisite, but none that I've found in the main book that have an unusual substance without a requisite.
The fallacy is confusing "if you use Muto, it's unnatural" (correct, because you bend essential nature), with "if it's unnatural, you must be using Muto" (incorrect, because there are many more unnatural ways to affect something, like destroying its ability to emit species).
What I find piling insult upon injury, is the idea that you can make something "survive" without continuous application of magic by applying, as a requisite, that Technique that always requires continuous application of magic. It's like saying that, because meat needs refrigeration to avoid spoiling, to keep something from spoiling in the absence of refrigeration you should add meat to it.
In addition to the fallacy in itself, I find this is making a lot of confusion between Forms, which capture what you act on, and Techniques, which capture how you act on it. So, if you want to create something non-mundane, it's a Vim requisite, not a Muto requisite. If you want to create something with Intelligence, it's a Mentem requisite, not an Intellego requisite. You put in a Technique only if you are doing stuff with the thing you create. So if you want to create a beast and control it, it's Rego. If you want to create a poison, and kill through it, it's Perdo, not because Perdo characterizes the poison, but because you are effectively doing Perdo-ing the victim of the poison by creating the poison. If you want to destroy a person's weight, it's Perdo Corpus, full stop, not Perdo (Muto) Corpus; but because it's an unnatural change, it stops as soon as the magic stops, just as it would happen with a Muto Corpus spell.
I hope I clarified the point 
Yeah I get what your talking about. Along the lines of how a lot of new players (and some older ones) try to tack a Vim req on to randomly because, "your doing Magic". I'm not sure the examples from TrME where really going there. The effects described where what I would consider in muto's ballpark, adding properties the thing does not normally posses. Mixing in plant or animal traits, and the appropriate Req's, to get a wall that heals itself. Or an ignem req for a wall that's perpetually aflame. And therefore creating something that lacked a natural property of stone like weight or the ability to emit species would then be Perdo.
Yeah the permanent aspect is pretty much my problem. Not so much the internal logic of the magic system but on the more practical where does it end level.
I can create a story in my head that explains why it works. Like Hermetic magic can't rewrite essential nature permanently but if you create ex nihilo your sorta writing your own code. You need Muto because that's the branch of Hermetic Knowledge when it covers those sort of changes. Form Req's are required when your bringing code from that another form. Blah Blah.
But then does that mean that I can mix the features of wheat and gold and grow a field of gold. It's essentially the same problem I have with the Create a Magical animal guidline. Is it any magical animal I know about (except critters of virtue those are specifically forbidden by latter rules), or can I make up animals with any magical ability I want. Again I've actually been in sagas where players actually created geese that laid golden eggs. Though admittedly the player who did it got the SG to establish that those where "real" known magical animals in our Mythic Europe.
Here's exactly where we disagree; or rather, I agree with your argument but not with the conclusions you draw from it.
There are two possibilities:
- What you create cannot exist naturally. Then there's no way to make it exist without continuous support of magic.
- What you create can exist naturally. Then just make it like that with Creo, no need for Muto.
Instantaneous Creo with a Muto requisite that "alters" what is created is plain silly, because the Muto part will end as soon as the magic ends, by the very definition of Muto.
What can, and cannot, exist naturally is of course up to the troupe and the individual saga. Is an invisible thing natural enough to exist on its own? If so, you can create it without Perdo. You are not creating it and then destroying its species. You are creating something without species in the first place: not having species is part of its essential nature, like being blind is part of the essential nature of some animals. But if you are creating something whose essential nature includes shedding species, then destroying those species is an unnatural change that cannot last without continuous application of magic.
Same thing with golden-egg hens, lighter-than-air stones, etc. Can they exist naturally? If your troupe decides they can, fine, Creo them with D:Inst, without a Muto requisite. Sure, they may require requisites like Terram for the hens, because a golden-egg hen straddles the line beween Animal and Terram. But not Muto; you are not changing them. If they cannot exist naturally you may still be able to obtain them through magic, but they will disappear as soon as the magic is over (but if you can obtain them in any way at all, you can obtain them though pure Creo)