Well technically, you'd be just as likely to have it be skinny then to have it be fat, b/c genetics determine that, whereas body size I would argue is more determined by lifestyle than by genetics (genetics does determine it somewhat, but not fully like it basically does with penis size... but overall body size definitely changes how skinny/fat it appears to be...
Like me, I'm 6'2", 170, healthy skinny, and mine's 7.25 in long and 6 around. That's pretty good thickness, but it's not insane... but b/c I have a naturally skinny body, it LOOKS more insane than it probably should.