There's only so much weight someone can put on before it becomes a hinderance as bodybuilders often don't make good fighters and are too bulky. Usyk obviously isn't a true heavyweight and I think he would struggle to get much bigger than he is right now but he should just do what he did against AJ and try outbox him and dodge whatever Fury throws at him. Easier said than done though and I think Fury is a better boxer than AJ.
Indeed Usyk is not a heavyweight. Joshua weight was 8kg more than Oleksanders. And Tyson Fury is (or was) 16kg heavier than Joshua.
Btw, comparing Fury and Joshua, Fury is more faster. I think this is due to Anthony being a heavyweight because he has pumped his muscles, which hinder more than help. While Fury has a lot of weight in his "life buoy ring". But frankly, Fury is to big for Usyk. Speed wont help him to avoid Fury's bombs.