This shortcut zooms out on the current worksheet, making items smaller.
Note: you can also use Control + mouse scroll wheel to zoom in and out on both Windows and Mac.
On Windows, the shortcut appears to be new with Excel 2016, and decreases the zoom level shown in the lower right of the worksheet by 15% each time Control - is used. Use Ctrl Alt + to zoom out by 15%.
On the Mac, there is no native keyboard shortcut to zoom in Excel, as far as we know. However you can use the following system level shortcuts to zoom in and zoom out:
Zoom out - Option Command -
Zoom in - Option Command +
Toggle zoom - Option Command 8
You can find these at System Preferences > Accessibility > Zoom