I recently jumped from an iPhone 4s running 8.1.2 to an iPhone 6+ and suddenly there is no more room for iCloud backups. When I go into "manage storage" I can see my old phone at 2.6GB and my iPad at 1.2GB.
My new phone was originally "restored" from my old phone, but my new phone tells me my next backup is going to be 4.2GB. This means I don't have room for one phone and one iPad in iCloud when I used to have room for just that.
Why is the backup suddenly so much larger, and what can I do about it? (My wife is also having the same issue although she went from 4s to a plain 6.)