The versions of pages themselves are stored in the database, not in the filesystem. Although lots of versions can make the database grow somewhat, we're only talking about a few megabytes here and there.

For any files that you have uploaded, they'll take up space in the /files folders. When you 'replace' a file, concrete5 keeps the previous version, it doesn't overwrite or delete the previous file. So if you have something like a 10meg file that you have replaced 4 times, you'll have 40 meg additionally taken up.

You can go into the file manager, click on a file and select properties. In there, there is a versions sections (I think it's a tab), that allows you to view and delete older versions of files.

There are some other folders that sometimes contain unneeded files. I think there is a /temp directory that is used to download updates to that can be cleared if it has stuff in it.

Also, when you use the automatic updater, it will unzip another concrete directory into the /updates folder. I think that after you have done a few updates, you can end up with several copies of the concrete system in there, but you only need the latest.(Don't hold me to this though, I update my sites manually - just something to look into)

There isn't much you can do to slim down concrete5 itself I believe - it's a complex system and it requires a lot of files.