I searched, but couldn't find anything, so here we go.

I've got an iPhone 16 Pro 128 gig, and for reasons I do not at all understand my "System Data" storage consumption is oscillating between 43 and 49 gb. Combined with iOS itself taking an additional 18 gigs, roughly 50% is being used by the phone itself, not me. And now my phone is pretty much full (I'm hovering between 124-127 usually). Obviously this isn't ideal and it's causing issues. Looking online, I have not been able to find any sort of remedial step aside from "iCloud backup and factory reset". I'm probably going to end up doing this, but does anyone here have any other ideas?

Incidentally, I already have my photos being offloaded to icloud (not happy about it), so there's not much photo storage. The biggest issue is that apps these days just take up more room than they used to.

Not sure, but I checked my storage to see how much my iphone (same model) is eating up. I use Brave as my default browser and noticed I had over 4 GB in "Documents & Data." Even after removing every cached item, the number didn't decrease. There was nothing in downloaded either.

I ended up deleting the app and reinstalling and that fixed it for now.

Suspect this is due to a) Apple not wanting you to use non-Apple apps and b) making the OS storage optimization purposefully buggy or sub-optimal to keep selling phones with more capacity for more $$$.

System Data is app cache so you have some apps in there that keep a cache that the iPhone doesn't group with the respective app. The best way to clean this to start a fresh install and restore backup from iCloud which nobody likes to do. The other option is to clear Safari cache. And try to figure out what apps have a cache clearing setting, I have a million apps so that would take too much time.

Nah, it's the developers that cache their data on your phone. But do look at how much iOS takes up. If you have a new phone, it's over 20GB because Apple has crammed in their 6GB AI model on your phone so you can use Apple Intelligence offline without sending your data to Sam Altman.