My guess would be that it's retaining your iPhone 4s backups. I had to swap my iPhone 5 over a month ago and even though I restored from backup and I kept the same name, iCloud retained the previous iPhone 5 backup and started a new one with the same name. I.e., iCloud had two iPhone 5 backups of the same name because it saw two different devices.
If your 4s had a lot on it to begin with and iCloud is retaining the previous copy, even though you may be using the data from your 4s on your 5 that could explain the lack of backup space.
You can check which devices are on iCloud by going to Settings>iCloud>Storage & Backup>Manage Storage. You'll see the devices listed at the top under backups. If you have two devices of the same name or you know that more than one device should be actually the iPhone 5 (now) then you can select it and delete the backup.
Hope that helps!
