I don’t think he will pack on incredible amounts of mass, to be quite honest. I think that would be incredibly foolish. Usyk can never match AJ, Fury, etc for sheer size but the salient point is that he doesn’t have to. He’s in another dimension to the heavyweights in terms of mobility and most significantly, speed. Fluidity of movement and mobility are so prominent in combat sports these days. One only needs to look at the current UFC middleweight division to see it. Robert Whittaker and Kelvin Gasteum, two former Welterweights who kept their fluidity and dexterity and have caused all sorts of problems for much bigger men. Cain Velasquez/DC/Stipe Miocic are all fine examples of it being done at Heavyweight too. Current WBC champ Deontay Wilder weighed in around 215lbs for his fight with Ortiz.
I actually think Usyk is all wrong for Joshua. It often gets disregarded but Parker did a fine job of making Joshua miss, he just couldn’t land anything noteworthy himself. I don’t envisage Joshua landing on Usyk, especially when you consider that Joshua’s own words directed towards beating Usyk were “let him tippy-tap and then explode.” Apologies, but I can’t see a generational talent like Usyk getting taken out by that. I also see Usyk being able to get in and out against Joshua with relative ease after 3/4 rounds to feel him out.
Whilst this is all hypothetical, I also think that Usyk is all wrong for Fury, too. Fury, by his own admission, says that slippery and smaller fighters are his kryptonite. Case in point, Steve Cunngingham. We don’t need to compare Usyk to Cunnigham, as that would be somewhat redundant, but you get the point I’m making.