Every couple months I have to trim various database tables -- anything non-essential to the site.
Posts/topics are essential. Members are essential. PMs, activity logs, and "Likes/Dislikes" are not.
So I go into the database and bulk delete the oldest 1000, the oldest 5000, or whatever number seems appropriate. I try to trim the tables down to 1000-1500 PMs, about 4,000 up/down votes, etc.
We used to have 15,000 up/down votes. That was only a couple months' worth! CathInfo is a really active forum.
I had to trim the database last night because I've been getting "database error" messages, which usually means the site is spending too much time getting stuff from the database -- my webhost will complain.
The larger the database, the more time everything takes. It's simple.