Well a "Factory Reset" doesn't actually totally clear off a device and start fresh. It will leave behind some things, so between that and things that may have been on it that shouldn't be, it makes sense. Also, 8GB total storage isn't just used up by your stuff, but it includes the space for the OS and all the apps and things the phone comes with. So you never actually have access to the total. And a "Hard Reset" would completely wipe the phone. Of all data and then reinstall only the original OS and Apps.
If, however, you were doing the "Factory Reset" to try and free up space, it really wouldn't have helped. What you need an SD card that you can move things like photos, videos, music, etc., to (not apps) and that would free up space.