We currently have eight new chicks that we picked up when they were less than 24 hours old. They they were about the same size the first day.. Out Buf Orps where twice the size of the other in a week . And now at about 8 weeks they are 25% larger that the SLW and GLW. And thought they weight much less than the Buffs our two Marans, the youngest of the new chicks, are even larger that the Buffs.
In answer to your questions,pasty butt could temporally slow down their growth and feathering. but more likely their breed their gene pools (and really a host of other factors) determine their size the speed of their development and later their longevity.