And now I feel smug that I've written several Harry/Neville and Remus/Neville stories, and he's always been described as not-thin. ;) He's round-faced, soft, and/or stocky, but he's not thin, and my Harry in particular tends to have a thing for bottoming for podgy!Neville. He gets off on feeling Neville's weight on top of him; it's like a comfort thing.