A word for changing 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."

Morph, metamorphose and mutate immediately come to mind ... And if he's changing the shape/size (in a negative way) of another object, mutilate.

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...

Bloat-man!!!