There is an action for that: https://docs.mp3tag.de/actions/remove-fields-except
remove fields except
Supply a list of fields that you want to keep. Enter their name without % and without any blanks. Separate them with a ;
I doubt that you will really save a lot of space as fields usually take up only a couple of bytes. And by considering the unavoidable padding, you will not see a lot of saving.
And any data that you delete is hard to restore.