My favorite is Dunlops. Dunlop is an American tire manufacturer, so you get the spare tire reference, but in the South, we say his fat's done lopped over his belt/pants. Needless to say, it's not the nicest comment.
Another one that no one has mentioned is some variation on love handles: ample love handles, big love handles, etc. Though, technically, the love handles are on the sides of the body, not the front, like a pot belly.