I get the feeling the phones and ipads just keep writing logs and diag files until they fill up when on dev betas. You could sync to iTunes/Finder which will, after a successful sync, delete all those logs (ready to fill up again).
I've only got a 16GB iPhone but an even smaller 5GB iCloud and the above worked for me.
How do you live with 16GB? I’m barely living on 64GB. My photos is 8GB, my songs are 15GB