my imac 21 " suddenly enlarges the screen

is it possible that you've discovered "smart zoom"? if enabled, when you lightly tap your mouse, the screen can zoom in or out rather unexpectedly. you can go into System Preferences > Mouse and check if "smart zoom" is enabled. if it is, deselect it and the unexpected zooming will stop.

however, if you want you can try getting used to smart zoom. once you get used to how it works, you will wonder how you did without it. :)

macOS has several Screen Zoom features.

see > https://support.apple.com/en-us/HT210978

and > https://support.apple.com/guide/mac-help/mouse-zoom-image-larger-mchlp2975/mac

or > https://alvinalexander.com/blog/post/mac-os-x/how-zoom-in-zoom-out-mac-osx-screen/