Think fast it multiplies.
You probably were not selective in copying files from the pb to the computer and did too broad a SELECT ALL. The Windows (mac?) has this uncanny ability not to be able to discern the "ANDROIDS COPIES" that are created or rather mirrored (or shortcutted) in the Misc/Android folder of things like photos, video, music etc. The pb create these duplicate REFERENCES but while the pb knows that they are not real, Windows does not realize this. So, if you copy stuff from the Misc folder, you MAY be copying the same files again. BUT beware - Android does create its own unique files in that folder so you might want to figure out if there's anything in there that is really unique.
And, by the way, while I knew this was the case, I, too, fell for this when I was doing a manual backup - had to abort it when I realized the issue.