It depends where you got your chicks. Hatchery chicks tend to be average in size and weight while breeders that specialize in a particular breed may sell birds that will be bigger and heavier.
I have two Light Brahma hens that are 2 years old. One is 7 pounds, the other is 5.5 pounds and they came from Meyer hatchery. They are friendly and let me pick them up. They eat alot and are messy birds when it comes to cleaning their roosting area in the coop. I have to hose off their rear end on occasion due to poop stuck in their fluff. They are good egg layers, not the best, but good.
Your ISA Brown will be an excellent layer.
You're going to have to get more chicks that are females. It is recommended one rooster per 8-10 hens. If you dont get more female chicks, the poor Brahma and ISA will be worn out by the Brahma rooster from over mating and most likely causing serious injuries to both your hens.
I forgot to add that I really like the Light Brahma's alot despite their minor faults. They are friendly and beautiful birds, eye candy.
