There's an expression "Like a greyhound ... all cock and ribs". It's enough of a phenomenon to have a saying arise from it.
In all seriousness, there may be a tiny bit of science behind it. Fat produces oestrogen. If you're a fat boy at puberty, you're going to produce less testosterone relative to oestrogen - the number of fat guys who were fat when young with small dicks I've seen is huge. I've seen plenty of little guys with enormous dicks. Having spent much of my life around elite sportsmen especially really big guys like rugby and waterpolo players, just because a guy is an enormous muscle god doesn't mean that he's hung like a stallion.
