I don't understand 5 GB for "backup". The smallest mobile device I have is 16 GB, so what does that mean?
iOS backups are pretty much only app data and system settings (not to be confused with the actual apps). This is because all your music and stuff are located on your mac anyway so a backup would be redundant. This carries over to iCloud where it is assumed that all your music and apps and stuff will already be in iCloud anyway since they're in the various stores. So this leaves the app data, i.e. if you make a document in Pages that will be backed up but the Pages app will not (because it is in the App Store anyway). Or if you have a game, the saved games will be backed up. This drastically decreases the size needed for a backup. Most apps have very little data that is backed up (some are quite large, such as Papers or other PDF reader apps).
So you'll probably be just fine as far as backups unless you have a lot of apps like that or a lot of iOS devices on one iCloud account.
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