The big one is slower feathering, and slower moving, spending more time than the others sitting down. It doesn't run around like the others, and generally seems like the leg muscles are too weak to support it, as if they haven't kept pace with the rest of the growth spurt. I don't remember there being any discernible size difference when we got them. Is this normal? Healthy? I'd hate to find out later there was something I should have done, if I'd only known...
That's a big size difference, sorry I can't help you but I know someone with more experience & answers will peep in. It's legs look like it should handle it's weight, it's possible due to it's size in need of vitamins .... Are your chicks from a breeder or feed store? 
