I think guesses on "average" breast size because of most bought bra-sizes are misleading.

I wear a 34 G and while I really couldn't run without a bra, I don't have any problems running or biking or walking with a nicely fitted sports bra. This "breast movement affecting running form"-thing somehow reminds me of old "scientific" studies stating that running women may lose their uterus because it would fall out caused by the heavy bouncing