Poverty and the offspring's size

Poverty doesn't arise merely from having babies but specifically from not endowing them with capital.

A not negligible part of capital is what we usually call morals.

Much of morals is learnt within the family.

A big family can perfectly be ideal for learning such values as parsimony, humbleness, collaboration, creativity, and respect to authority.

Poverty has much more to do with not teaching values to children than with the offspring's size itself.