SSD on OSX Mountain Lion says it's almost full but isn't
TRIM isn't enabled on non-Apple OEM SSDs in OSX.  Perhaps garbage collection is not happening.  You can find a lot of sites that explain how to enable TRIM in OSX.  I haven't provided a link as they are plentiful and you may wish to use a site you're more comfortable with.  Use keywords: "enable trim support OSX terminal"

This link explains it but you should not need the freeware tool that the article mentions.  You can enable it from terminal with a few commands.