Every Live Photo is a photo and a short video, and your Mac keeps both

iPhones take Live Photos by default. Each one is a still image plus a short video, about 1.5 seconds either side of the moment you pressed the shutter. The video part is often as large as the photo, or larger. Over thousands of pictures, that is a lot of disk.

How much they add

So a library that is mostly Live Photos can be close to double the size of the same pictures as stills.

Count them on your Mac

In Photos, the sidebar has Media Types, Live Photos, which shows how many you have. To see the videos sitting in the library on disk:

find ~/Pictures/*.photoslibrary/originals -iname '*.mov' -size -10M | wc -l

Short videos under 10 MB in the library are mostly Live Photo clips. Terminal needs Full Disk Access to read the library.

Turning them off for new photos

1. On the iPhone, open the Camera and tap the Live icon so it is crossed out.
2. To make that stick, go to Settings, Camera, Preserve Settings and turn on Live Photo. Otherwise it turns back on next time.

What about existing ones?

In Photos on the Mac you can open a Live Photo, click Edit and turn off Live. That changes how it is shown, but Photos keeps the original video in the library so the edit can be undone. It does not save space.

The only way to remove the video from existing Live Photos is to export the stills and import those instead of the originals, which loses the Live part for good. For most people, turning them off going forward and using Optimize Mac Storage is the better trade.

More on the library: Photos library taking up space.