Chicks that Free Range

I bought some 2 day old Cream Legbars & 4 day old Exch Leghorns earlier this year, kept them in a brooder, fed them the right food etc etc   before I lost half my chickens recently (to infectious sinusitis)  At 14 weeks they were doing ok  but seemed to take a while to 'grow'  so much so I contacted the seller to check, anyway I lost them .   My broody bluebell hatched 3 chicks, 2 welsummer crosses and a buff Orpington which have free ranged since birth.  They are healthy, happy and, at 12 weeks old, look very mature and 'plump' I have never given them any special food, they eat what they find and what I have put out for the others.   Does this mean chicks that free range develop quicker & stronger?