[DaltonST]

When did you last "check library"?

Calibre > Library > Library Maintenance > Check Library.

Doing so automatically vacuums/compresses the Calibre database file, metadata.db, which can grow enormously over time as compared to the optimized, compressed size. Checking the Library also checks it for corruption, etc.

After compressing Calibre, defragmenting and compressing your hard drive is also a good idea. SSDs don't need that, but do need periodic trimming to optimize their access speed.