You haven't gotten many responses yet so I'll just throw this in here as an idea:
I wonder if the problem has to do with how you downgraded to Catalina; specifically what volume you erased and installed Catalina on, and whether there might be one or more Big Sur volumes left over (accounting for the missing space). Both Catalina and Big Sur use APFS and create multiple volumes on one container disk (which is mapped to one physical storage device). All of the volumes can share the actual capacity of the physical drive. With APFS it is entirely possible to have two "250GB - seeming" volumes on one 250GB SSD. Of course you can't store 500GB of data, but each volume will look like it can hold 250GB (even though it can't if the other volume is taking up some space).
I believe Cataling and Big Sur create a read-only system volume and a user data volume. I wonder if one of those (now un-needed) Big Sur volumes is still there, taking up space on your physical storage device. I believe this maybe can happen if during the downgrade you erased and installed to one of the volumes instead of erasing the top-level storage device.
I'm sorry I can't give specifics -- I'm still running Mojave and can't be sure exactly how your SSD volumes should look in Terminal or in the Disk Utility.app, or whether Disk Utility has changed from Mojave (hint: in Mojave one must go to the View menu item and select "Show all Devices" in order to see the top-level physical storage devices).
If you post a screenshot of Disk Utility (with "Show all Devices" turned on), someone might be able to tell you if it looks OK or not, for a Catalina install. Hope that helps.
