When I share my screen, the screen is shown as much smaller than it should be. What can I do?

The issue is most likely caused by your display’s resolution. if you’re using a 4K display, you will need to manually lower the resolution of the display to fix the issue. You can read more about changing your display’s resolution on the following links:
Windows: Change desktop icon size for screen resolution - Microsoft Support

Mac OS: Change your Mac display’s resolution - Apple Support