Cron takes care of cleaning the files, I'm not sure whether it does this every hour or once a day. Yes, this is normal because you would have tons of files that aren't needed -- as the name suggest they are temporary, and can be safely deleted after some jobs are done.

@morfik, @question
Actually there is one more, its oc-content/temp folder and oc-content/uploads folder that may hold temporary images.
These should be cleaned with hourly & daily crons. It may get bigger here if a lot of pictures was uploaded.