Mac storage basics · Photos, music and video
Why is my Photos library bigger than the photos in it?
Short answer: It also stores edited versions alongside originals, thumbnails and previews, Live Photo video parts, analysis data and shared-album caches. Don't remove them by hand.
Besides your originals, the library holds edited versions (Photos keeps the original and the rendered edit), thumbnails and previews, Live Photo video parts, analysis data for search and People, and caches for shared albums.
None of that should be removed by hand. To make the library smaller, delete photos and videos you don't want (then empty Recently Deleted), or use iCloud Photos with Optimise Mac Storage. See iCloud & iCloud Photos.