Someone who's roly-poly is small and round. Your fat little Corgi puppy is extra adorable because she's so roly-poly.
A chubby baby, with dimpled elbows and fat little legs, could be described as roly-poly. When you use this word as a noun for a plump person, it's especially derogatory, but the baby bears at the zoo won't mind being called roly-polies. When someone talks about a roly-poly bug, they mean a pill millipede or a woodlouse, a small insect that can protectively roll itself into a ball.