Hm, if messages really take up space then it could be. I have never deleted any texts since I first got an iPhone, about four years ago... I mean literally every single text is still on my phone.Could it be SMS or MMS messages or call history?
What iOS version are you running? I was having this problem on my 4S and iPad Mini, each taking up a gig of space. No camera roll. This was on 6.0. I think it had something to do with SMS/iMessage history not wiping properly and the only way to fix it was wipe the iOS device and start new.
But either 6.1, 6.1.1, or 6.1.2, not sure which one, fixed this problem, and my iCloud backups are now at manageable sizes. They both total now a gig in size, which is where I think it should be.
It's all the backups for the iPhone in iCloud. If you really want to save space backup the iPhone on the Mac instead.