He is a stocky guy. AJ might be taller but not as stocky. Has skinnier legs, lower bodyfat. Ive seen smaller and shorter looking guys than Itauma who weigh the same. Everyone carries weight differently.
Combination of being heavy set and carrying a bit of extra weight. Tyson Fury being huge is a myth. If he ever got in shape he’d be built like a 220lb basketball player. Has almost comically skinny legs. Broad shoulders and longs arms create the illusion of size. Joshua is deceptively light because his body fat is so low, but he also trends more towards lanky than heavy set. Someone like Joseph Parker is more like Itauma, he was literally an in-shape 267lbs still being called a ‘small heavyweight’ compared to the lanky blokes.