The iPhone cannot be backed because there is not enough iCloud Storage for that Where can I determine what apps make the incredible size of backup?



As you can see there is almost 30GB free space, but I cannot perform a backup because there is not enough space on iCloud. Why?

Using iOS 11, iPhone 6S Plus 128GB

Under Manage Storage > Backup there is following screen:



My question is different to the marked one as duplicated, because:


I have 50GB storage on iCloud including 30 gb free space
using iOS 11
my problem IS NOT about why backup still have 5GB, but why there is no space, even when backup is 20gb (what is rather impossible).
You can temporarily increase your iCloud storage ($1 per month) and do your backup. You can then check how big the backup is. I noted you have 128GB on your phone. If only a fraction of that is used it could be a lot. After your backup, you can reduce your icloud storage back to 50 GB to end the $1 per month charge.