Why does my imac 2024 desktop screen suddently enlarge as I'm typing.

We need to see what all is running, a report from this will not display any personal info...

Using EtreCheck - Apple Community

EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info. Give it Full Disk Access.

https://www.etrecheck.com/

Give etrecheck Full Disk Access before running.

Thanks for Old Toad’s etrecheck instructions…

Slow iMac 2017 - Apple Community

Go to Apple > System Settings > Accessibility > Zoom and review those settings.

If those settings do not fix...? Click Advanced... and review those settings.

Does this happen with any app or just one?

Are you using an Apple Magic Mouse? If so that could be the culprit. On mine, there is a Mouse setting where a double-tap (not double-click) zooms the screen.