Does whatsapp backup media on Google Drive?
If you see that size is not matching, it's likely that you didn't tick include videos in WhatsApp settings





If you have ticked it and it still doesn't match, that's strange because I have restored to test, when starting to use this feature and all media plus charts are backed up. 



  does whatsapp keep previous backups on Google Drive ?


No

WhatsApp FAQ says :


  Every time you run a Google Drive backup on a Google account, the previous Google Drive backup will be overwritten. There is no way to restore an older Google Drive backup.


So if for some reason your backup doesn't include videos after ticking it also, it makes sense to take a fresh backup on WiFi