Mouse pointer enlarges for a moment I have an iMac running macOS High Sierra. Sometimes, when I'm moving the mouse pointer around it will enlarge to 2-3 times the size for about 1 second. It happens a few times per day.

I assume I'm either pressing some key that causes it or I'm moving the pointer over something that triggers it.

What is causing this behavior? Is there a way to disable it? If yes, how do I do so?
The behaviour that you are encountering is due to an accessibility setting called Shake mouse pointer to locate.

First introduced in OS X El Capitan 10.11, it helps in locating mouse pointer for people with vision disparity. It is also widely used for easily locating the mouse pointer when using a multi-monitor setup and/or high-resolution display(s).

The setting can be enabled/disabled by going to System Preferences → Accessibility → Display.