Chickens have a size limit like all birds. Science has tried growing bigger chickens like the Cornish Cross but they get heart infections and deformities in their legs because they grow so fast. Their butcher weight is reached in less then 8 weeks. I raised a few and felt so sorry for them, the Cornish Cross grows so fast it can't walk but the time it is 5 months old.

If you have noticed all land animals have a size limit, no matter what scientist do they cannot excrease a species size beyond a certain limit.