This still does not explain why these panels grow, but maybe it has something to do with how Lazarus handles scaling on retina displays.
I don't have a Retina resolution monitor and it happens to me on Big Sur
Same here with a Mac Pro. Probably Big Sur related?
Curiously Apple's applications do not suffer from it
defaults delete lazarus.freepascal.ide
Ironically, after I updated to Big Sur 11.1 beta, I think Apple must have fixed this issue anyhow, as I can no longer get it to go wrong in my older app built with Lazarus 2.0.10 running on 11.1