X-Chat logs are HUGE (not that it's x-chat's fault in-particular). I have a big harddrive, but i'm not going to let some log file steal 20 GB of space.
(20 GB is an estimate, but it was HUGE, I was shocked to see how much space it took up. That was only for like a few months too).
Channels and private messages take up a lot of space. If you have logging on, you might want to reconsider staying idle in a channel all the time :\.