What determines the weight of a baby? For example, all three of my chi…

What determines the weight of a baby? For example, all three of my children were born at 4000 kg, the last one when I was pregnant, I didn't eat wheat for nutrition, I watched that it wasn't bulky. Anyway, there was 4 kg weight. I gained 12 kg during pregnancy and 4 kg weight of the baby, sometimes some gain 20 kg and the baby weighs 3 kg, just curious what determines this? Write your opinion or observation you have.