But they are hybrids and cannot be expected to breed true in any case. I would expect them to get heavier and heavier and become unable to walk easily or stay upright, in large part, as the Cornish-Rock crosses do. We have 105 (or so) FR on pasture at this time, 8.5 weeks old, straight run. Lotta roosters starting to crow that funny cracking-voice crow.
We found one that could not walk last Monday night when we went out to shut them into their "hoopie trailer." It was very thirsty and I'm guessing that it had probably not walked for most of that day, behaved just like CxRs whose knees lock up or hurt too much or whatever it is that makes them stop walking. It spent the night in a box in the garage with some water and became dinner on Tuesday, dressed out at about a half ounce over 3 lb (just coming up on 8 weeks, and not the biggest chicken in the bunch.)