Where did you get them?
My first thought is pullets, because of the darker overall coloring. The (sorry, don't be offended) poor quality barring is often seen in hatchery birds. Then, I noticed the bottom bird doesn't have a straight comb, not sure if it's rose or what. So, is it a Dominique? That could explain the size difference, as Doms are lighter bodied birds than rocks. Or, are they mixed breed birds? In which case you can't sex them by color and just have to wait a few more weeks for the combs to tell the tale.