Josh, most stuff I've read has said as your weight goes up most of your lifts will go up too. But at a certain point if your weight keeps climbing your Deadlift will start to stall, there are various reasons for this, the belt not allowing you to get down is probably one of the reasons.
Anecdotally I have noticed the same thing on the few occasions I reached 220-230lbs bodyweight, bending over and picking up the bar was considerably more of a struggle than when I'm my usual 190-200lbs.
I don't think the ab-size is a major factor here, unfortunately
Sorry bro, I don't mean anything bad with this but I think it's a bodyfat issue.