iPhone 8 has calendar data of c. 40GB - same calendars in iPad is 4MB -all fixes tried haven't worked

I have an iPad and iPhone 8 - both current OS versions.

With the same calendars, the iPad calendar data takes c. 4MB, and the iPhone c. 40 GB.

I have tried removing all calendars one by one, without any difference.

I have removed the calendar app but it retains the events which seem to be the issue.

I tried doing a factory reset - which was problematic - as it would start to reinstall the OS then hang, but complete when rebooted (by leaving it to shutdown due to low battery).

When it initially finished reinstalling, it had c. 27GB of events - in the past day or two, this has increased to c. 40GB.

Is there a way I can look at the backup data to try to sort out the issue - or is there another way to resolve this?

Many thanks,