If you're running Ubuntu Mate and your Zoom suddenly has huge buttons, it might assume you're using a very High DPI display. While this is usually a good idea, the heuristic determining whether to enable this HiDPI mode or not does not work perfectly with Ubuntu Mate and multiple displays.

Force a specific scale factor

Unfortunately, this setting is not exposed in the GUI, you'll have to change the config file.

First, make sure zoom is closed completely, i.e. there should not be a zoom icon in the panel next to the clock.

Open ~/.config/zoomus.conf and search for autoScale. Make sure it's set to false.

• If you want smaller buttons, set the scaleFactor parameter to 1.
• If you want bigger buttons (because you have a HiDPI-Screen), set scaleFactor to 2.

When you re-open zoom, buttons should have the desired size.