What it says on the tin: I tried editing a Sword save file with version 20.06.19 and it threw a filesize error, specifically stating that the file was too large, listing it at 1575632 bytes. In the attached image, a Shield file that I successfully edited is on the left, while the Sword file that threw the error is on the right. The edits made to the Shield file carried over correctly.
I had had a hunch that it was due to saving while in the Isle of Armor, but trying a save on my Sword version in Wedgehurst also threw a filesize error. I have not been to the Isle of Armor on my Shield file yet, so it's possible that accessing the DLC area and/or getting the pokedex upgrade is the culprit. Otherwise, I'm stumped. Any thoughts would be appreciated!