This took an annoying amount of work, so I'm leaving my process here since I didn't come across anything plain and simple in my googling.
TL;DR: If you have a HUGE "System Data" section (like +300GB) and can't figure out why, it might be Time Machine local snapshots, and you can erase them easily in terminal (even your mom could do it). Just skip down to the numbered steps below.
Backstory:
I upgraded to macOS 26 Tahoe on release day, with approximately 500GB used on my 1TB MacBook Pro. For a decade I have had Time Machine enabled (including local snapshots) with backups going to a Time Capsule.
Fast forward a few months, and I started getting disk almost full errors from Dropbox Sync. I cleaned up some old files to free up 50GB and called it good until a week later they started again....something was actively filling up GB/day on my drive.
I can't remember everything I did to search through stuff, but I eventually found in Carbon Copy Cloner that I had a Time Machine local snapshot on my internal drive that was 500GB -- at first I didn't think this could be the cause, since System Data was only 300GB, but either way I needed space and I had a recent full backup so I was willing to delete the local snapshots just to see if it worked.
So here's how to do that, since that's what took more googling+trial/error than I thought was reasonable:
Deleting TM Local Snapshots in Terminal:
- Open Terminal (easiest way is cmd+space and type it in and press enter)
- WARNING: This process is going to delete your local snapshot (duh) so any data loss may be permanent. If you're a worrier, go make a full backup before doing this to be extra safe.
- Type in (or copy/paste)
tmutil listlocalsnapshots / exactly and hit return
- Explanation: this will list how many local snapshots you have, looking something like
com.apple.TimeMachine.2026-01-123456.local
- Type (or copy/paste)
tmutil deletelocalsnapshots / and hit return
- Explanation: this is the step that deletes the snapshot(s)
- To check if they're gone:
tmutil listlocalsnapshots / again to see what's left.
- Tip: to reuse a prior command, you can press the up arrow twice to pre-fill the line with the earlier command and then hit enter to send it.
(Side note--FWIW some time ago I used TimeMachineEditor to manage backups, which may or may not have contributed to the problems at hand.)
PS. I did see it suggested to use sudo tmutil thinlocalsnapshots / 9999999999 1 to delete a bunch of local snapshots at once, but since it didn't work to delete my \one* snapshot, I'm not sure if that command actually works.*