We live in the era of nukes. Australia which has 1/50 of China's population can construct 200 thermonuclear weapons for military parity with China. You do not need millions of people. Those wars are over,A big country needs a lot of people. More people = more power. Pakistan need to increase its population to match Indian troop numbers in case of mobilisation.
Ideally, Pakistan need to overtake USA on population numbers to become the third largest nation to secure its future in the 21st century.
21st century will be the century of the mega, and giganations.
In 30 years, Pakistan's population will reach 400 million people, and it will be the point where it will overtake USA. If USA's population growth will slow further from the current 0.5%, this will happen even earlier.
The black swan event here which is very obvious, but not talked about is the possibility of immigration flow reversals.
It may now sound ridiculous that any brown country will see a positive migration, and even more unbelievable that a currently wealthy Western country can lose not only its immigration allure, but see a population drain, but this is already happening with Chinese, and to part wealthiest part of Indian diaspora. Here on this forum we see even Pakistani members sometimes anticipating good times ahead, and keeping one foot back in Pakistan just for such scenario.
Countries with high populations of recent 1st, 2nd generation migrants themselves have much more mobile populations. People who come to wealthy counties for money, may well as easily leave them for money, and opportunity too.
It does not matter what Pakistan's population is. Right now they cannot impose their will on Iran or Saudi Arabia. The only neighboring country that it can impose its will is Afghanistan. Even there it is fighting for 4 decades. Forgot about Pakistan competing with India or China. You can see even with China and India. India's parity in population with China does not mean equal economic outcomes. With their current level of human development 400 million Pakistanis might make no difference in grand scheme of things. You ignore environmental degradation the current populations have on their countries. Is any Muslim country looking at Pakistan as a role model for anything ?
I understand your concept of immigration reversal. Not every society is as open as America. Even China might not be accepting of large scale inflows of Chinese migrants back of China. A few numbers here and there is a novelty. I can see 1st and 2nd generation immigrants keeping options open. 3rd, 4th or 5th generation Chinese immigrants in America are not going to China.