> As the population has grown poverty has decreased.

This is like comparing apples to oranges to find a correlation, and arriving at an incorrect causation.

Falling populations are largely due to better education, specifically of women, as well as integration of women into the workforce, which has been one of the bigger drivers in curbing poverty. The latter is so significant in the west and the far east (double income households mean rising costs of living, meaning SAHM households become unviable in the West for the middle class), resulting in a significant population decrease.

But in places such as the Middle East or Africa, ever increasing populations have only meant even more financial trouble for those economies. Case in point Pakistan and Egypt. In other economies such as India (which I assume is one of the "couple countries" you mentioned), overall birth rates are falling significantly to near replacement level, and in the few states/regions with high birth rates, poverty is rampant and the people reliant on subsidies from other more prosperous regions.

The problem with most of the world is that while we are able to sustain a huge population as a whole globally, most of the population growth is extremely localized, resulting in population sparsity in some comfortable regions (Canada, the US, Central Asia, southern Russia) and population unsustainability in other regions completely unsuited to population growth (sub-Saharan Africa, Indonesia, the Middle East).