It really doesn't matter because, I can easily have the birds exercise in the run the weight that is put on is fat...do you have any ideas as how to do so.
I *had* two banty cockerels/roos (not sure on age) that were dumped over my fence. When I caught them their breast bone was razor, razor sharp - almost concaved.
I feed my flock (chickens, ducks, geese) flock raiser - 20%, a my scratch mix (50 pounds oats, 50 pounds commercial scratch and 50 pounds black oil sunflower) plus house scraps. The dumpies, I feed like my own, and just processed them today (2 or 3 weeks - will have to look). Do not have a before and after weight, but breast bone was alot less boney when felt.
After plucking I can see both have the beginnings of fat deposits and some meat - I mean they are banties.
I don't know if this is helpful at all.
eta: I do not feed 150 pounds at once - sorry - but throw a handful or two per bird in the pen. (I have small hands)