Of course Usyk looks thicker. He's 3 or 4 inches shorter and has a smaller frame so his mass is more localized. With Wilder it's more evenly distributed across his larger frame. As a result Wilder is way leaner looking (more functional lean muscle mass).
You do realize that Usyk used to be a middleweight for a while in the amateurs when he was younger? He fought at just 165 lbs. That's why Fury laughs and calls him a "middleweight" all the time. Wilder's lowest amateur weight when he was younger was 200 lbs (heavyweight in the amateurs). Let's not pretend that they're naturally the same size. They're not and it's clear to anyone with a working brain.