Usyk handled plenty of heavyweights in the amateurs, I've watched his fight with Joe Joyce and the size difference wasn't an issue at all.
I know its only sparring but Dave Allan said Usyk hits as hard as anyone he's been in with.
Then you've got the gulf in boxing ability, the bookies have Chisora as 7/1 which is pretty big for a heavyweight clash, I think there's a reason for that.