When you go to Settings -> iCloud -> Manage Storage -> Device Name -> What does it show as backup sizes? You have to do this on each of the devices to get the correct amount. I'm assuming its trying to backup more than you think it is.
Thats the current backup, when you actually click on the device name, it will take you to another menu that will take a minute to populate saying "Next backup Size". In order to get to that screen, you must click the iPhone on the iPhone and iPad on iPad.
Screen will look like this:
If you see that spinning wheel like in the screenshot on the left of the image, its still calculating the size.