All is relative, I guess. But SD is more than double the size of Milwaukee in population. San Diego County is the fifth-most populated county in the US, and it sits next to Orange County, the sixth-most populated. I work in Orange County a lot, and the drive down to San Diego is no more than when I drive from Madison to Milwaukee (I know, Cali traffic can make it worse.). I've made the drive, leaving Irvine after work to be at a Padres game. Not all small markets are created equal.
And I agree that putting Miller Park in a huge parking lot really takes away from it. I am getting close to my goal of visiting every major league park, and there is nothing like the vibe around those downtown parks. Take urban transit or Uber downtown, don't worry about parking. The scene is lively around there. Love it. San Fran, Colorado, Washington, St. Louis, for example. People walking around, bars, restaurants. Compare that to sitting in a line of cars trying to get off of Brewer Way.
Edit: Well, shoot. Those posts saying almost the exact same thing as what I wrote didn't show up until I posted. Or I'm bad at internetting.