Yes there is automatic removal so if you have enough disk space, no need to worry about it.

What triggers this? My DB was about 30GB, then the update shrunk it down to 5GB, then it grew back to 30GB.

The pictrs storage only consists of uploads from local users, and thumbnails for both local and remote posts. Thumbnails for remote posts could theoretically be wiped and loaded from the other instance, but they shouldnt take much space anyway.

I'd be pretty confident that the 140GB of pictrs cache I have is mostly cache. There are occasionaly users uploading images, but we don't have that many active users, I'd be surprised if there was more than a few GB of image uploads in total out of that 140GB. We just aren't that big of a server.

The pictrs volume also grows consistently at a little under 1GB per day. I just went and had a look, in the files directory there are 6 directories from today (the day only has a couple of hours left), and these sum to almost 700MB of images and almost 6000 files, or a little over 100KB each.

The instance has had just 27 active users today (though of course users not posting will still generate thumbnails).

While the cached images may be small, it adds up really quick.

As far as I can tell there is no cache pruning, as the cache goes up pretty consistently each day.