Tellers could have gotten twice as efficient and still double in size if the number of transactions requiring human intervention increased four-fold, especially if the move to online banking has driven an increase in fraud and user error. It's a number that scales with regulatory demands, transaction count, total volume, and so on - not number of active users.

Edit: Tellers also tend to be general purpose account managers at smaller banks, so if you called up the bank you'd eventually get forwarded to someone on the floor at your local branch instead of a call center.