Deleting FileStream data doesn't reduce Database size

I have a database with some FileStream data. I don't need the FileStream anymore and I want to delete them to reduce the size of my database.

(The way I'm measuring the size of my database is I'm looking at the .BAK file produced when I back up the database to a file).

I've deleted the records but this did not reduce the size of my .BAK files at all.

I read something about garbage collection and tried to force this but couldn't figure out how.

Is that the problem? Shouldn't garbage collection occur automatically? It's been 2 weeks.
Does deleting the records containing filestream data really delete/reduce the files on disk containing this data?