I solved it.
I have to installations of MC. One in my PC and a portable one.
I needed to edit from my PC the database I had at the portable drive. (I needed to do this because there was a bug at the portable version. I can't remember right now what it was).
So I backed up my library from the portable drive and loaded it from my pc.
The problem is that when we keep files at the same drive, that we have installed MC, their path appears in the library as "(Install Drive):/" .
When I deleted some of those files, MC erased the filepath, but didn't erase all the other information (i.e. Name, Bitrate, Bit Depth ...). Even when I tried "Import" where I hoped it will fix the broken links, it didn't. It just reimported the files, as (Install Drive):/ . As a result my database was growing as I was trying to delete files.
The work around was to change the (Install Drive):/ to the specific portable drive letter. (pick songs->right click -> move, copy... ->Update Database to point to new location -> Find and Replace -> c: to x:). We need to be careful when writing the drives at "Find and Replace". MC won't recognize (Install Drive):/ as a drive letter. We need to place there the name of the drive where the MC we use at THAT moment is installed.
I guess that (Install Drive):/ was introduced to help people with portable installations. I find more convenient another way. That is, before installing MC I chose a rare letter for my drive (X:/) and every time I mount that portable drive to a new pc, I assign again that letter.