I have a local library of a little over 25,000 tracks, with files organized like this: artist folder / album folder / tracks.
I used this convention for the name of the track files: DD-NN track_name. DD is the disc number (if applicable) and NN is the track number.
I saw that the path and file name are stored in the Roon database, so I wanted to see the impact of reducing the length of the file name. I removed track_name from the file name and left only DD-NN.
After that, the duration of the backup was reduced by 30%. Also, its size has decreased. 
I suspect that something similar happened with the database size.
Note: I did the renaming of the files with Roon running and it did not affect the identification of the tracks at all.