So I downloaded 11.1 beta for iTunes and it made me dig around in the options for the first time in ages. I noticed I had a crapload of backups stored from old iPhones, iPads etc... since like 2 years ago. Deleted them and it's still in the process of getting rid of them, so I'm watching my HDD space skyrocket... turns out I had at least 70GB of backups, if not more since it's still going.
Why on Earth does iTunes do this?
Edit: Finally finished. It was about 80GB in total. Crazy.
It will keep one backup for each device you have. However, each time you do a restore, it will append the backup with a time and date, copy it, and start again.
If you do a lot of restores (and some of the major iOS updates actually didi a restore), then you will quickly pile up on those large backups.
Mac OS X has a habit of utilising disk space in excess to make things 'convenient'. For the most part, I'm kind of appreciative. I know a lot of people get annoyed that disk space is being used up, but if it is for convenience, I don't mind. A good example is how iPhoto manages non-destructive editing by having a master copy and only editing on a copy, essentially doubling the storage taken up by photos.
That said, it does suck if you don't know stuff like that is going on, and Apple COULD be a bit more open about some of it and provide tools to clean up old unwanted copies. for iTunes backups, you can obviously do it through the preferences.