I get the next backup 0 bytes too but it seems to be backing up. iOS 9 beta had issues bit 9.1 seems to be working other then the next reported backup size. If you have Windows PC you can run the iCloud control panel and delete the old iOS 9 beta backups that were hosed from there. That worked for me with the betas.it shows 0kb for phone backup but it backs up photos etc without a problem.
When I hit delete phone backup it wont let me so i can start over.