Strangely enough, the name of the glass preceded the name of the quantity within.
Two tablespoons, in American Colonial times, was the commonest amount for toasting. This was most often drunk from daintily mouthblown goblets, as befitted an expensive distilled (or fortified) drink. The problem was, it had become fashionable to slam the glasses on the table, to mimic gunshots. (It was that, or throwing them into the fireplace...) This meant a lot of expense for the tavernkeeper.
However, this didn't mean that they couldn't fight back. Some quick work with some expert glassblowers yielded a cheap molded glass, with a heavy, curved, bottom that made a gratifying thump when slammed. When filled to the brim, they made handy measuring cups as well.
Now you know.