What kind of magic would it take to make a fat person slim?
Is it...
...Perdo Corpus, for the obvious reason?
...Creo Corpus, because he'll be nearer the ideal?
...Rego Corpus, because it's a natural state of the body?
...Muto Corpus, because obesity is part of Essential Nature? 
(And don't say Muto Imaginem!)
I'd say muto corpus, perdo would be difficult since you'll probably affect his total weight/volume. Rego is kind of weird, unless you'll control his metabolism rate, but then there is paradigm. Creo could also be an option. It depends if you'll want slim people to be the ideal version or think a persons appearance is part of the essential nature. Since I don't like the way ppl idealise slim people id go with muto.
Creo corpus to bring the body closer to ideal would work.
Perdo corpus to make the body undergo the damage of starvation would work but would have accompanying deleterious effects.
Rego corpus to make the body drop weight over a period of time (perhaps duration moon or duration year) would work.
Muto corpus would work for the duration of the spell. A muto corpus spell to give the character a body that needs three time as much food to satisfy could work if it were accompanied by not givign the body as much food as it needs.
I could see an enchanted device with an intellego corpus spell determining when the body has had an appropriate quantity of food acting as a linked trigger to give the target a whak to disincline them from further consumption or even a perdo animal/herbam effect to destroy food int he target's stomach.
Getting outside of corpus
a perdo herbam/animal spell to diminish the nuritive properites of any food that hte character eats would also work.
Muto mentem to change emotions of hunger into fear or a desire to help others
Muto imagonem to make all food smell horrific when touched by the character
pedo corpus to destroy the character's sense of taste
Creo mentem to make the character feel full
There's dozens of ways to go about it.
The most straight forward way would be a creo corpus ritual.
My Verditius has already considered an item to help such a thing, which would be ideal to sell to overweight nobles ^^
The trick isn't to just make them thin, which would require a ritual and not be worth as much money, but make an item which lasts for say, a year or so, which just Creo's up some sun duration food.. it will let them feast as much as they like, but with no long term nourishment effects.. as long as they're instructed to have one real meal a day, it'll literally be the perfect diet, where they can gorge themselves as much as they like and still lose weight 
Not 100% sure on the requirements, but given the old, nobles mostly eat meat thing, should probably be Animal primary, with requisites of Herbam (for vegetables or fruit) Aquam (for various drinks) and Ignem (for cooking). Starting at base 10 for summoning the corpse of an animal, and probably throwing in a +1 magnitude for flexibility, as in the enchanted chef in covenants, and maybe a target of part, given the user may want various sizes or pieces of meat.
The end result as I can tell would be..
The Basket of Plenty
Level 55 effect CrAn (He/Ig)
Range: Touch Duration: Sun Target: Part
Base 10, +1 Touch, +2 Sun, +1 Part, +1 Flexibility, +2 Requisites, +10 levels Unlimited uses per day
The basket of plenty can create any kind of food within itself that the owner can imagine, whether raw or pre-cooked. The food created by the basket is only temporary, and while it sates hunger, does not provide lasting nourishment, rather than being detrimental, this is marketted as it's main advantage, allowing the owner to gorge him or herself on the fantastic contents without gaining a pound of fat.
Mainly made with an effect expiry of 1 year, meaning a lab total of 61 is enough to create the item in one season.
That seem alright to everyone?.. unsure, because it has a few requisites and got a little complex... I decided to ignore the Aquam requisite in the end, soup is overrated, and giving nobles the ability to create endless wine would perhaps be troublesome.. though it could certainly be added at only a +1 magnitude.
man! I would kill for the real-world version!
I wonder, though, whether being obese is really a huge concern for nobles. Wouldn't it be a sign of prosperity? Or have people pretty much always hated obesity?
A question and a comment:
Q: is the obesity a Flaw? The I would consider how to 'police' the ease of magic your or others way out of it.
C: Beside from many things the stigma of being fat is not an universal truth - so it might not necessarily be something you'd feel urged to want to get rid of. In generel terms that is - of course any character might for many reasons feel different about it. Just don't forget that in societies of want there's often a lot of status in being nice round and fat.
Depending on the exact period, in historical times being what we today would call "fat" meant you had enough to eat, and were therefore attractive to some. To use "Creo" would create the ideal fatman, a jolly and attractive Bachus or Falstaff. Kings and the rich were fat and envied for it - no need to lose weight to be the ideal. (Unless you want to argue that one could Creo the "ideal" of anything - the ideal leper, the ideal cripple, the ideal dwarf, etc etc.)
To even suggest Perdo is not to understand it, imo. The very first line reads "Perdo makes things worse examples of the kinds of things they are." When it then says "Perdo could remove a person's weight", it means "the fact that they weigh anything", not "slim them down nicely." Unless you simply want to remove "a pound of flesh" here and there, the destructive nature of Perdo would only leave a very unsatisfactory gaunt, weakened husk, I'd say. (Especially since the whole concept of lyposuction is a few years out of paradigm.) ymmv, Dr. Munnigant.
Rego could act as a corset, but I'd be hesitant to say it would actually "thin" a person.
Muto would be the way, imo. You're changing one shape into another.
I don't speak with any authority here, but I guess that for a nobleman being fat was not a social handicap -it could be viewed as a sign of prosperity.
But anyway, a noble might rather be not-so-overweight, since that could hinder his combat ability (and thus unable to attend tournaments, for example). Admittely, this would arguably be more a problem for a younger noble.
In any case, I would consider that the 'ideal' human shape existed -that which was represented by greek and roman sculptures. I would suggest CrCo magic would transform somebody closer to that ideal, effecively transforming them into a walking Michelangelo's David!. It could even work for somebody who is slimmer than the ideal.
You're not thinking Rube Goldberg enough.
Your character already has healing spells available to him. So rather than invent/invest some new ritual/formulaic/enchanted device to make someone thin (a one use spell that you'll never need again) instead you invent a Perdo corpus masterpiece "Touch of three years famine". You use it on the noble and then heal the poor boy back into health.
You get the job done
The noble won't come back for more favors unless it is very important
You have a new combat spell
You didn't have to invest vis in a ritual
work smarter not harder
So almost kill him with a starvation spell, then use Vis to permanently Creo him from dying? Interesting... a bit risky, perhaps- not every noble could survive 3 years of famine. 
But yeah, that could work, or something like it. I'd invoke some sort of risk of killing the target (maybe not right away, giving the mage time for the follow up spell), so it wouldn't be pleasant. But there are many "1 spell and then 1 more" solutions to problems- a PeCo spell alone doesn't solve it, that was my position.
Banishment of Lifes Excess
R : Touch / D : Mom / T : Part
Level 15
PeCo
All excess fat is removed from the targets body.
As Perdo Magics cannot make a thing a better example of itself ,
the target is left with many folds of loose skin.
A Muto spell to change the appearace while the body sheds the excess skin naturally may be required.
A Creo spell doing the same thing will require a ritual spell and Vis use.
(Base 05 / +01 Touch / +01 Part)
If Base 05 can destroy a corpse , then it should be able to destroy something like body fat.
As the target loses all body fat , they can possibly be considered the equivalent of lightly wounded ,
because some body fat is needed for physical comfort.
Just to be clear, my original question was strictly academic - I have no "need" for the spell, I was just curious as to how it would be implemented.
If I were to actually create a spell for weight-loss, I'd make it a Mentem Spell (as someone suggested) since it is more subtle. Make a potion of it and sell to royal ladies to slip in their husbands' drink (the ladies, at least, may appreciate fat and prosperous noblemen less than the peers of said noblemen). Shouldn't be too much work, even at Moon duration.
Incidentally, what's the Form bonus for an actual potion? Or can charged items not recieve Form bonuses?
I had already designed the spell for the Level 10 - 15 spell thread.
I'd definitely say Muto. Technically Creo does make something into it's more 'perfect' form, but I dont believe that covers a subjective aestheticism such as body-fat percentage!
Don't be silly, vis isn't for NPC's! Use the month duration recovery boosters.
(this statement is intended to be tounge in cheek)