you will only see posts/comments/upvotes that happen AFTER you've subscribed to a community
when you subscribe to a community it will pull the most recent posts made but it won't pull the comments or upvotes for them, just the post itself
this is why a lot of instances use https://lemmy-federate.com/ but that will use up a lot of your disk space
It will increase over time, and I think it would pass 100 GB in less than a year, but I'm just guessing. You can help reduce this by tweaking some settings with image caching/proxying