Hm.. I see what you mean, but that horse has never been fat actually.. I've always had trouble keeping his weight! The first summer I had him, he lost too much weight, he looked awful.. :/ And that was on a 24hours turnout on several acres with grass.. x_X; I've learnt to keep his weight good now tho, I think and hope.
Could the belly be a lack of abdomen muscles to keep it up or something, you think? (first picture)
I hope he'll look better later on :P Or else I'll have to reconcider how I ride. x)
Throwing in a picture from when I bought him 2 years ago (tho it's the wrong angle to see much..):
And how he looked that first horrible summer, before I figured what to give him to get his weight back...I tried everything.. x__x I think he had started gaining some at that picture, actually..