To me, a young fellow who looks "well fed"'s hard to put into words but for me it's a vaguely uncomfortable vibe. A feeling of teetering on the edge. The definition of well-fed is hard to pin down, and I don't want to sound dismissive. It refers to the physical look of someone who's not fat or plump or chubby, but who seems to enjoy eating. Someone who's just a tiny bit heavier than he/she ought to be.