They found out that the small (16 oz) was actually giving you more beer (20 oz) The small cup was not to be filled to the brim like in the video because it gave you more beer than you should have.
They apologized to the fans and showed that the price they were charging for was for the correct volume for the large, but the small was giving you a lot more.
We received a copy of the YouTube video showing the quantity of liquid served in the 16 oz. beer cup is the same quantity that fits into the 20 oz. beer cup. This is the first time we have been alerted to this fact. Upon our internal investigation this afternoon, we discovered the cups that are marked 16 oz. hold 20 oz. of liquid. Fans who purchased a 16 oz. beer actually received 20 oz. of beer for the 16 oz. price. Fans that purchased the 20 oz. beer received the amount they purchased.
Seattle PI Link