OneDrive - Can't delete .pst file

The size is growing because every time outlook touches the file it counts as a change and it’s resyncing another copy every single time it’s opened and closed etc. so this space is due to the version history copies.

Best I can guess you are not closing out of the outlook process when removing the pst from the OneDrive location. I would suggest removing the pst from outlook profile. And closing outlook. Wait for sync then remove the file.