according to SD, the jb jr actually has a lower frequency peak, although not much, so it might actually be fatter. however, it is wound slightly cooler. the coils are closer together, so this would result in more phase cancelations, or high and low end trimming. and it sees a more narrow section of string, so the harmonic content would differ. instead of a bridge humbucker, which has good harmonic content over a wide range, the jr would have excellent harmonic content in a short range, and suffer slightly in others. the magnets are also different, the jb is alnico v, which is what pafs are made out of, and the jr is ceramic, which is what just about every passive metal or distortion pickup is made out of (but that doesnt mean a ceramic pup cant sound great clean).
in short, there are too many variables to know for sure. i dont think of the jb jr as a minature jb pickup, because of all the differences, but they may in reality sound similar, only one way to find out 😁
