If you receive this message then your diagram file in Google Drive is nearly full. Google imposes a 10MB size limit on the file types we use, but that includes the full history of the file, which we're unable to delete.

The quickest workaround in this case is to make a copy of the file using the drive.google.com interface and continue working on the new copy, that will have no history (you may wish to delete the old file).

We'll continue to investigate whether we can improve this process for users.