Since medievally, shadows(and cold) are as real separate "things" as light(and fire), i dont see a point in specifically going "modern knowledge" just for this.
Even today its totally normal to talk about how "those shadows are moving" or similar. Nitpicking against the paradigm seems rather a bad idea.
Back more to the original topic, I've been mulling over this idea for a little while again. Should this work with MuAn for a leather bag, etc.? First, though not an argument specifically against it, the conditional statement doesn't work that way. If you can change something you can make an image of such a change does not imply its converse. But neither does it negate its converse. From the flavor perspective, I see something very fey in the feeling of a road that just seems to keep going and things like that, so messing with space fits well with Faerie Magic for me. Again, this doesn't invalidate the MuAn approach. But something still bugged me...
Now I know what was bugging me. Note that this is not a pure rule thing but rather my interpretation of the rules in what I think is a very reasonable manner. First I'll start with a totally different example to set the stage. If you turn someone's blood to water that does not give them the ability to use water as blood, but rather they no longer have the blood they need. The Muto effect on the blood does not change body that needs the blood. Now how does this apply to the bag? If you use MuAn to make the inside of the bag bigger than the outside you do not affect the space, just the bag itself. Now let me show you how that is actually very reasonable. With a real bag the inside surface is actually slightly smaller than the outside surface. Imagine such a pouch being inverted. Yes, you can do it, and yes the inside is now bigger than the outside. However, the bag must get distorted to allow such a geometry - you get all sorts of folds and the like, depending on how far off the areas are. That's what I see happening with the MuAn effect. The issue is that you want to have a spell affecting the space itself, not the bag so much. Space is all about perception. (No, I'm not going to use my very modern understanding of relativity, even though that might help.) Even people way back when understand that space varies by perception from practical experience. Take a spear fisher, for example. They know to aim below the fish if they want to hit it because the space below the water is deeper than it appears to be. So I see this affect itself being an Imaginem one because it's all about how the species reach you. Do you have to reach a longer than expected distance to receive touch species? Do the light species have to travel a longer than expected distance to get to you? Etc. So to me vanilla Hermetic magic could make the space inside of the bag appear to be larger than it is using MuIm (and I think there may be a similar example or two in canon), while Glamour could make that appearance a reality.
Chris
Conjure (from RoP: F) is clearly described as allowing this type of effect. Could that be what you're thinking of?
A way around this, which one of the characters in Magi of Hermes uses, is items which transform things put into them. So, he has a bag which turns large things put into it into small things, and then carries those small things. So, thousands of gallons of water turnd into teeth, and he carries the teeth.
Not a perfect fix, but well within Hermetic limits.
Similarly, there's magic to shrink people, so a tent to house 35 is easy if you shrink the people to be 1" tall each.
That is it exactly, thank you Gremlin. I thought I was losing my mind. 
I actually had this come up in a game I ran, I posted about it some time ago. My ultimate ruling was that I couldn't see a reason why it couldn't be done. The wizard in question ultimately regretted the expansion of his top floor lab when the floor could no longer hold the weight of his possessions. It also made way to some fun stories. Ultimately making any item hold more than it should in going to be a huge vis investment as you have to reinforce the item in question and any other creative problems the troupe can come up with. Not to mention the item warping in strange ways. With RoP: Magic you could have all kinds of fun with a 200 year old bag that tries to eat people. Plus imagine someone looking over your shoulder in a tavern when you open a bag containing a room.
Anyway, I decided it was Mu and was a usually the most extreme of the magnitudes (making something highly unnatural etc.) given an object. Each size boost increasing its size by one step on the Lab chart. For objects teat it like a growth spells.
Another note, this means you could make an item hold less than it should, and hilarity ensues....