User directory taking up an additional 100GB+ of space

Solution 1:

Take a look at Time Machine, it may be keeping local snapshots.

From Apple Support:

Because Time Machine removes local snapshots as needed, Finder and Get Info windows don't include them in their calculations. To see how much storage space local snapshots are using, choose About This Mac from the Apple menu, then click Storage. The space used by local snapshots is labeled Backups.

You can disable them, but they get automatically deleted if you need space:

• If less than 20% of the total storage space on your startup drive is available, Time Machine removes local snapshots, starting with the oldest, until you have more than 20% free space.
• If less than 10% or less than 5GB of storage space is available, Time Machine removes local snapshots more quickly. When only one snapshot remains, Time Machine stops creating new snapshots. As free space increases, Time Machine at first replaces the previous snapshot with a new one, then eventually resumes creating snapshots as normal.

Solution 2:

It was the Time Machine local backups. After running:

sudo tmutil disablelocal

I have my space back! The /Volumes/MobileBackups/ folder was enormous.

I never use Time Machine aside from restoring from a catastrophic failure, and I don't know if I've ever touched the Time Machine GUI - so the local backups were completely unnecessary for me.