Unfortunately that is how you have to backup exchange servers. Yes, it is a lot of data. Purging must be done regularly. Also your public folder containers need to be well maintained.
One option would be to execute a daily differential backup with a full backup once each week. This could be good or bad for you depending on the nature of your work and the way in which data is stored on your exchange server.
If you don't backup both your mailboxes and your information store, you simply won't be able to restore them to your exchange server.
I can give you more details on how we have set up our exchange backups if you require?