If they are free ranging, that should be plenty of space. I also don't think space issues caused this by any means. Overcrowding issues initially display as behavioral issues, feather pecking, etc. Not by illness. Illness comes later.
In answering all the questions I came up with last night, you did not give me anything environmental that I could hang this on. Where I agree with
@Shadrach is sadly the breeder. How has she reacted to the information that you have lost yet another? You have told her, right?
With sudden death like these my mind immediately goes to genetics. That is however a likely by-product of my business and education.
I also think that the sooner you get them outside the better. Once my chicks were fully-feathered they went outside. They need to adapt to their real environment as soon as possible.