How can system memory take up so much space? I mean most S3 phones are only 16GB total.
Something is obviously amiss but not sure how to get to the bottom of this. Any help or suggestions would be appreciated.
That's definitely not right, if I was in that situation I would try to make a backup in recovery (if system is actually 20GB I doubt you'll have enough space) and then I would wipe everything (system, data, cache, and dalvik) thoroughly in recovery and reinstall the ROM to see if this issue persists.
If you go to Storage, in settings, and tap on the various categories shown there (Used Space, Miscellaneous files) you can dig down and maybe see what is taking up so much space. Give the phone a minute or so to calculate the space each time you go to a new page.