WHAT HAPPENS WHEN the small get big? Craft beer is at a turning point. While upstart breweries continue to blossom—more than 150 opened last year—the companies that started the movement two and three decades ago have grown into relative giants. The largest by far is Boston Beer Co. Started by Jim Koch in 1984, today it operates three breweries and produces 1.2 million 31-gallon barrels of beer annually—so much, in fact, that last year the Brewers Association raised the definition of "craft" from a yearly cap of two million barrels to six million. This jostled a few industry pint glasses. After...