It's simple.. Kevin obviously used less GH/Insulin.
And it's not only his gut, but forehead, elbows, which seem perfectly normal even after retirement. Kevin actually looks good for a 40 year old considering what he had to put his body through. On the other hand, guys like Ronnie, Dorian and Nasser will look oddly distorted unless they stay on HRT for the rest of their lives. Imagine them off gear.. They'd have huge heads and stomachs in proportion to the rest of their bodies.
Say what you want about Kevin. But atleast he can look like a normal, healthy and decent looking individual out in public. Can't say this about any of the other greats.