this is a tough one to answer. because iTunes varies what it actually backs up. things that are synced from your iTunes library aren't in the backup. so if you sync music, video, movies, tv shows, they won't take up anymore space on your hard disk
please see the link
https://support.apple.com/en-us/HT203977
my backups tend to be large like 7 gigs iCloud.. iCloud does not back up as much as iTunes does. i like to keep iMessages forever so my backups are big
my ipad folder is 7 gigs and my iPhone folder is 30 gigs. but i think thats because iTunes keeps a long history of incremental backups
that support article will kind of say that in certain situations iTunes won't backup your camera roll or maybe if you use photos and transfer them to the library, then iTunes won't take up more space by putting them in the backup?
what you do is complete an iTunes backup. then you can go to iTunes , preferences, click on devices. on that screen you right click on the backup set. and you tell it show in finder. once the finder window opens, click on command I
whew
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these screen shots can help you see how much space your backup can take
[doublepost=1460408471][/doublepost]both macs , my mbp and the mac mini ( which is used more frequently to backup ) both show the iPhone ( a 128 gig model ) takes 30 gigs
[doublepost=1460408575][/doublepost]my 64 gig iPod touch , which has 50 something gigs of sync'ed music, only has a 1.6 gb iTunes backup