By far the most space is taken up by votes. You can disable votes (1.0 will have more options), or delete them directly from the database.
Yeah I guess the issue is that votes are the thing that makes Lemmy what it is. Without them it’s just a stream of posts with no ranking. Hard to avoid needing that space.
It’s not a huge amount of space, but if you’re on a cheap VPS then they don’t come with much space.
You could write a query to votes for remote posts older than 6 months, or something similar.
Or instead of individual votes do a dump like an rss. Like only update when I ask every 10 minutes or so. Get everything from x - y time and just give me the totals. Probably only good for smaller instances though.