Check out “Screen Sharing Preferences” at > https://support.apple.com/guide/mac-help/share-the-screen-of-another-mac-mh14066/mac
2 Choose Screen Sharing > Preferences, then set the following options:
- Under Display, select “Scale to fit available space” or “Show full size,” depending on how large your screen is and how much of it you want to use for the shared screen. This option changes the default behavior for when you share the screens of computers you’ve never connected to before. To change the current scale, choose View > Turn Scaling Off or View > Turn Scaling On.
I don’t have a 13” MacBook to test this with. But I also think that if you change the MacBook’s display resolution, then you should be able to get a larger image of it on the iMac’s display.