Thats because its is effectively a two-child policy in most parts fo China. If your first born was a girl, you can have another child but you have to wait 3-4 years. If your first born was mentally or physically disabled, you can have another child. Also note that ethnic minorites such as Uyghurs and Tibetans can have 3 or 4 children and are not included. The one-child policy is enforced at the provincial level and not at the national level so implementation varies across China. The Chinese government estimated that only 35.9% of the population today are still affected by the policy and many people simply refuse to abide by it so today. But they were able to bring birthrate down from 5.8 to 1.6/women which is pretty impressive. All across the developing world the birthday is usually double that of China's birthrate today.
Yeah but where did that heavy industry get us? we still can't even produce large cargo planes, large airliners, aircraft carriers, modern cars/trucks up to western standards, modern automotive engines, efficient turbofan engines/stealth technology for next generation fighter jets. Our steel industry is behind India's and we have to import subsystems for our military hardware from Russia/Israel and Ukraine. Basically when we were suppose to worry about feeding our people, we were developing nuclear weapons and building an army like North Korea today. Feeding people is more important to standing up to the world. I'm glad that now we're on the right track but I can't imagine what China would be like today if it had 300million more mouths to feed today under the same circumstances.