Backing up whatsapp to two different google drive accounts
Whatsapp store last 7 backups. 

You can see it by checking your file manager -> Whatsapp -> databases and there, you can see almost 7 files.

The size of files vary from each day.

Since you are changed your google account, the last backup file only be stored in G Drive and previous backups will not uploaded there.

So, after 7 days, you can see that the size in G Drive becoming 841 MB.