Why is the population of Pakistan so much larger than that of Iran?

Historical factors and economic factors.

1, historical factors: Pakistan has a relatively short history, while Iran has a longer history. In the past few centuries, a large number of immigrants poured into Pakistan, which made its population increase rapidly. In contrast, Iran's population growth is relatively slow.

2. Economic factors: Pakistan's economy is relatively developed, and its domestic market and foreign trade have a certain scale, attracting more people to live and work here. Although Iran is also an economic power, its economic development level still lags behind Pakistan.