The representation of capitalists (and some politicians) with a large belly probably dates back a couple of centuries, and the underlying idea back to antiquity. Originally they were representations of men with 'enormous appetites' (and not just for food!), and who had the means to satisfy those appetites. If you were powerful, you weren't 'fat' - you had 'gravitas' (Churchill is a fine example). Skinny men were viewed as weak; and not without cause, since the laboring classes were chronically malnourished, and would be represented as wasted beggars.
This material condition continued to exist until quite recently, and the representation continued as a convenient short-hand (the meme ossified into a thought-fossil).
Ironically, as Dot pointed out, now the underclass subsists on industrial regurgitations and we balloon in weight, while the svelte ruling class dines on organic vegan chicken breast and hand-picked quinoia polenta before attending their monthly liposuction appointments. (Note to self, do not get me fucking started on Quinoia!*) Now the belly is more associated with greed in the powerful, and with weak-will in the rest of us; though i think shadows of the original fossil still haunt our minds.
And you will note that the above meme concerns _exclusively_ men! Women had their own shorthands (or something.)
*This really pisses me off. Another example of the upperclass foodies stealing a staple from another culture, completely without context, and their aspirational drones flocking along behind... And as a result, the people who ate the stuff for centuries can't afford it because every wannabe from Manhattan to Beverly Hills has a pantry full of the stuff. And some rich bastard got richer trafficking the stuff, and the people who subsisted on it now have to subsist on the same industrial shit we have to because they can't afford anything else. None of which has anything to do with the question, or my answer. It just really fucking pisses me off. Thank you for not listening.