Hi!
BigSur has some compatibility issues in general, so it can be it. But also if you are using a mac with a high resolution display it can be an issue as well. This is a known issue with retina displays, I experience it too, the only solution to this is to reduce the resolution before starting Kali, you can do that using an app called “resolution changer”, download it from the app store, set the resolution to 1080p and it should work. Or use any other app that reduces the resolution size.

Greetings!
Diego