An important step for fast & efficient development is a demographic window. That essentially means a country will have a lot of young people, and therefore a prominent workforce, capable of providing for the "dependents" (children and elderly). Assuming 2.1 children per woman is necessary to maintain stable population levels, to achieve the demographic window a nation would require some good 3 - 4 children per woman which is the case, as of now, in Africa. (Also, many of you
Also, in regards to poor people having a lot of children,
It is hypothesized that the observed trend in many countries of having fewer children has come about as a response to increased life expectancy, reduced childhood mortality, improved female literacy and independence, and urbanization that all result from increased GDP per capita, consistent with the demographic transition model. The increase in GDP in Eastern Europe after 1990 has been correlated with childbearing postponement and a sharp decline in fertility.
In advanced countries where birth control is the norm, increased income is likewise associated with decreased fertility. Theories behind this include:
- People earning more have a higher opportunity cost if they focus on childbirth and parenting rather than their continued career
- Women who can economically sustain themselves have less incentive to become married.
- Higher-income parents value quality over quantity and so spend their resources on fewer children.
Missing that demographic window has some costs, and so does inorganically (and carelessly) decreasing fertility rates.
My argument here isn't advocating poor people having a lot of children, as such behaviour does have perverse effects (birth mortality & criminality rates & overpopulation in urban areas & some others) but instead that overabundance of people isn't a burden to development (in fact, as I pointed, it's the opposite of that). An underdeveloped country with stable population (for instance, Brazil) will still have terrible life conditions due to its underdevelopment, and said underdevelopment is produced by a multitude of factors, especially those related to productive capacity & institutional strength. Those two things are strongly tied to long-term goals or, as we Brazilians call, a "nation project", a set of cross-generational development projects, which is often the opposite on what happens in underdeveloped/developing countries (where populist, short term measures to alleviate the immediate misery are the norm).
Lastly - and that may trigger a lot of people, but it's undeniable - historical inheritance is another heavy factor for underdevelopment. The constant meddling of world powers (the three largest culprits are always US/UK/USSR) in internal affairs has delayed a political interest in developing a "nation project" in several, SEVERAL countries.