He is certainly doing it on purpose, mate. His whole fighting style is built around trading blow for blow and overwhelm his opponents, for which being the bigger/heavier fighter is very determinant.
If he slimmed down to 195lbs, as you suggested, first of all Im not sure he can do so, because he is a thick dude with a big frame, but if he does, he would not enjoy such a physical advantage over his opponents, which is his main asset. He would still be a tough guy but not sure if a top one.
I would favour Uriah Hall in a rematch vs a 195lbs Costa for example, based on what we saw in their first fight, where Hall showed better technical ability but eventually was overwhelmed by Costa's pressure and power. Im not sure Paulo could do the same with 20lbs less behind his strikes