Outook.pst does not decrease!

I have a user that was unable to send/receive emails due to her outlook.pst file being too large. I created an archive.pst file so she can move all unneeded emails to it and reduce the size of the outloo.pst file. She moved everything and the archive.pst files is now 1.6+GB in size, but the outlook.pst file did not reduce in size, it’s still its original size. Any suggestions?