I looked around and I don't think any other questions explore this specific issue, but a lot of stories use transformation magic and I find it confusing. In this situation specifically I'm referring to, say, a human, who can turn into a cat (like Professor McGonagall) as an example, which is a creature that is of a smaller mass than adult humans. I suppose you could look at this kind of transformation like you look at the TARDIS (bigger on the inside), but what about if, for example, a human transformed into a dragon? In this case, the dragon is much larger than the human, so where does the mass come from? You could just say "magic" but I guess I'm looking for a more solid explanation, even if it doesn't scientifically make total sense - it is fantasy.
And one more thing that stumps me is where do their clothes go? Professor McGonagall's robes disappear with her body when she transforms into a cat. In some stories I know that when a person transforms, they lose their clothes or rip them. However, I'm not fond of the idea that every time my character transforms into a dragon, their clothes are destroyed and when its time to switch back they're suddenly naked. Maybe an enchantment, or is there a better way to explain it?