Backing up WhatsApp to two different Google Drive accounts

I have had my WhatsApp account backed up to my personal Google Drive account, taking up a total space of 841 MB. I now have access to a Drive account with more space, and I'd like to switch the backup to there. I tried changing the settings on the phone to back up to the new account, but I only get 549 MB there. Why would the two backups be so different? I'd like to delete the original one to free up space, but not if it means losing data.