iCloud iOS backup says not enough space
I'm trying to backup my iPhone to iCloud via wifi. Currently I'm using 28.7 MB of 5GB total, so I have plenty of space. I'm not backing up Photos or Mail, the two largest items. However, when I go into iCloud backups and hit Backup Now, it eventually tells me, after a loooooong time calculating the time remaining, that I don't have enough space.
No detail is provided, such as how much space my backups require or what is taking up how much space.
Any suggestions how I can get this backup to work? I'm loathe to throw money at Apple for a larger iCloud account when I don't believe I need one.
iPhone XS