Massive difference in file size

Hello everybody,

I just intended to backup my databases what I usually do in a .zip file to have them all in one place.
This morning I realized how long it took, so I took a closer look.
Now, there is one database which is shown to be 6.5 GB in size.

Whereas the contained documents only sum up to around 2 GB

Is there a point I am missing here?
Thanks for helping me out, best regards,

The database contains a lot more stuff, e.g. backups, metadata, thumbnails & search index. The total size of the contents can be found in File > Database Properties.