If you look at market size for a baseball team based on the number of people in the area in which your team would be the natural “local” favorite team, it only stands to reason that the Brewers would be in one of the smallest markets.
Many metropolitan areas that are similar to Milwaukee have other cities in their natural areas of fan interest. Cincinnati, for example has the closest major league team for cities like Indianapolis, Louisville, Dayton, and Columbus,
The Brewers natural area is less of a circle than a compressed semicircle because of Lake Michigan to the east and Chicago to the south. That geography gives the Brewers a lot fewer people who would naturally be fans because of geographic proximity.
I don’t believe that the Brewers market of fans is anything other than one of the lowest in MLB.