Your guest's view of the presentation may be displayed larger than it should be if the presenter is using a Macintosh computer with a Retina display and the resolution is set to one of the higher (scaled) resolutions.

To correct the issue, the presenter should either pause or stop presenting, change the display resolution to Best (Retina) or less and then start presenting again.

Note: Never change the screen resolution while you are presenting (sharing your screen). Doing so will produce a distorted or garbled view for the guests.
If this happens, stop presenting and then resume presenting to correct the garbled guest view.

Mac Retina display setting dialog. Use the first 3 settings for best results when screen sharing.