Your backup is stored in iCloud - not on your iPad. The local storage on your iPad and your iCloud Storage are entirely independent.
You have a free 5GB iCloud Storage quota. You can subscribe to additional iCloud Storage tiers - 50GB, 200GB and 2TB for a small monthly fee. If your iCloud storage is full, you have two choices:
1) Reduce the storage that you use by deleting data - or eliminating the iCloud backup (recognising that eliminating the backup is perhaps unwise)
2) Subscribe to a higher iCloud storage tier.
Manage your iCloud storage – Apple Support
iCloud storage plans and pricing – Apple Support