I own an ipad pro with capacity 256GB. I have been trying unsuccessfully to perform an icloud backup. The next backup size comes up as 699.96 GB - almost 3 times the size of my ipad capacity - what is wrong with this picture?

Ensure that you have updated your iPad to the most recent version of iPadOS - currently iPadOS 15.5.

If your iPad has been updated, perform a forced-restart of your iPad - as so doing will usually clear an isolated software issue:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

After restart, revisit your iPad storage - and verify storage usage by category:

Settings > General > iPad Storage