Hello Nils1406!
Thanks for reaching out here on the Apple Support Communities! I see that you're Activity Monitor and Mail windows are able to be resized, but when you quit the app and return to it, the window size is reverting back to the way it was before resizing it. If this is correct, let’s begin with some basic troubleshooting, and then work our way toward more complex seps. This will help us to isolate the issue further with the app windows resizing:
1 - Quit all the apps you have running.
2 - Restart your iMac: Go to the Apple menu in the top left corner of your screen > Restart.
3 - If the issue persists after restarting, restart again. This time you'll boot into safe mode and test again: Use safe mode to isolate issues with your Mac.
4 - If still occurring in safe mode you’ll need to create a new user and test it out again. This article will show you how: How to test an issue in another user account on your Mac.
If you still are experiencing the same issues with the app windows not retaining the size you've changed them to, or if you’re seeing any error messages, let us know. Also let us know if it is isolated to Activity Monitor and Mail or if there are any other apps affected.
Cheers!