I did some searching, and while I found a few threads about mismatched bearings, none really described what I'm looking at tonight.

My rear wheel is an A-Class ALX200. It's been spinning pretty rough for weeks, which surprised me because I've only had the bike for 6 months / 2000 miles. Tonight I pulled the bearings out, and on the drive side I found 9 shiny 1/4" ball bearings with clean cone and cup, as expected. On the left side I found 11 rough 3/16" ball bearings and a pitted cone.

I'm going to take the cone+axle+bearings to the LBS tomorrow for replacement, but has anyone else seen mismatched bearing sizes in a single hub before? How likely is it that it's supposed to be 1/4" bearings on both sides, and that this is the cause of the pitting? I've tried visually comparing the two cones, and they do look a little different, but it's hard to tell if it's just the pitting that's giving me that impression.