I am not sure how many Buff Orpingtons you have seen, or what their breeding was, but not all Buff Orpingtons are the same (just as not all White Plymouth Rocks are the same). I agree that Buff Orpingtons are loose feathered, and White Plymouth Rocks are closer to a hard feathered breed (though not as much as say a Cornish or Malay), but some Buff Orpingtons are massive even under the feathers. I don't know of any Plymouth Rocks (white or otherwise) weighing 12-15 lbs. (not in the hands of a normal fancier anyway). Yes, I know that is above standard weight, but some Orpingtons (particularly Buff) can attain those weights. I have a fairly large Buff Orpington Cock (around 12 lbs., though he might be a little larger now), and it is interesting to see him next to the Plymouth Rock cockerels. Granted, it seems to take an excruciatingly long time for the Buff Orpingtons to fill out, but once they do, some really are massive, even aside from the feathers. I have a son from the aforementioned cock, and I thought he was a pullet for the longest time, until he began to fill out. He is still filling out, at around 9 months of age. As far as the thickness of legs, there are some Buff Orpingtons with thick legs, though they tend to be short in proportion to their bodies compared to a White Plymouth Rock.
As far as female White Plymouth Rocks not showing the effects of being bred by their feathering, I have seen many instances where they did have their feathers messed up by mating, but it was generally by an aggressive male.
