I wonder if the 6oz beer is partially an attempt to speed up service. When pouring beer, they often have to wait for the head to go down, which can slow down the amount of time before someone gets their food. I can only assume that a 6oz beer would need less resting time than a 12oz or 20oz. Since wine and mixed drinks don't need to rest, that could explain why those pours are unchanged
The Brewer's Collection booth was added a few years ago to assist the Germany booth, which has historically had a high beer-to-food ratio and a slow queue to match. Germany continues to sell a lot of beer (though presumably less than in the past, with the Brewer's Collection next door) and the queue continues to move slower than at booths with less of a beer presence
Additionally, it makes it easier to forecast demand a little. If there's a long line, you can start pouring a couple beers in advance, knowing that everybody will get the same size