Does Rebel Wilson not get pissed off that even in CGI cat form she's a lardarse?
If she lost her weight, she wouldn't be a funny, outlandish, cuddly, bubbly lovebug that she gives off and no-one would employ her stereotype. In time, when she gets bored of being a stereotype, she'll go to a diet farm, lose that weight, tell everyone her true name and age, and try to be taken as an actor/actress. There will probably be a story in there of the fat-shaming she's had to put up with and she'll eventually end up on Big Brother when people catch onto her.
I know James Corden lost a lot of weight when he... lost all that weight... But mainstream America will still see him as a British John Candy. He's a different beast as I think he was an 'actor' first, and sort of became a funny-man through the years.
Point being, the fact that these two 'celebrity performers' have been cast in Cats shows you how desperate they are to appeal to as wiiiiiide a range of audience as possible.