Turns out that all "puberty: You're doing it right" posts in response to Matthew Lewis's butterfly-like transformation during the premiere of the 7th Harry Potter film were slightly faulty.
Matthew Lewis didn't actually bear that much a resemblance to the buck-toothed, chubby gryffindor he was play. According to Matthew, he wore a fat suit for the first six Harry Potter films.
Additionally, he had false teeth and bits of plastic attached to the end of his ears to make them stick out. All his clothes were too big, and they even had him wear shoes that were too sizes to big for him because Neville was described in the books as having long feet.
Interestingly enough, what seemed like Lewis's physical transformation paralleled with Neville's emotional transformation. He went from being the petrified, toad-losing boy we saw in Sorcerer's Stone to an epic leader in Deathly Hallows.