According to 1Password documentation, the .1pux format is just a .zip file. I would suggest changing its extension to .zip, then using File Explorer to explore the .zip file, and delete the contents of the files folder, which is probably what is making your file too big. Then change the file extension back to .1pux and attempt the import again.
Before you do the above, make a backup copy of the .1pux, because it contains your attachments (which will not be imported into Bitwarden).