For an assignment this term I have to come up with an anti-cliché superhero. I am going to do a superhero that changes shape from thin to big and was wondering if anyone knew a word for this change of shape/size.
Consider protean, "Exceedingly variable; readily assuming different shapes or forms"; "From Ancient Greek Πρωτεύς (Prōteus), the Greek warden of sea beasts, renowned for his ability to change shape."
It sounds like you want a synonym for horizontal expansion (like puff or pop) more than you want a word for shapeshifting, since your superhero's morph abilities seem pretty specific and limited.
In any case, an anti-cliché superhero ought to have an anti-cliché name. How about Mister McPuffins (his tumescence is swell!)? Or Mary Poppins's cousin Percival Poppins?
Following Urbycoz’s gist, how about SupersizeMan? (def. supersize)
In a more humdrum vein, to increase in size is to augment, and Augmentor has a kind of X-men feel to it.
A "superhero that changes shape from thin to big"?
Dinna dinna dinna dinna...
To answer the more general aspects of your question, the basic term for changing shape is metamorphosis. Which also means that names based on the body types noted above have a useful intrinsic allusion in place.