A bunch of stuff - all of the metadata images, posters, previews, and analysis Plex has gathered and performed for your media. It’s made up of many small files that can take a long time to transfer/copy/back up.
https://support.plex.tv/articles/202529153-why-is-my-plex-media-server-directory-so-large/
Some people turn off features entirely if they don’t use them, or if they need the space.
It’s also possible to reduce the size of Video Preview Thumbnails significantly while keeping the feature active.
What’s a rough order-of-magnitude estimate of how many Movies and TV Episodes you have? 45 GB isn’t particularly big if you have a significant number of movies and shows.
