Do most poor people actively want a lot of children, or is it rather something that happens to them while having sex? Either because they don't have money to use contraceptives, or are too stupid to use them properly, or simply at the moment of passion they forget to think about any related things... In other words, what exactly is the causal chain which creates most poor children? The answer could be different for different groups of people.

Or, perhaps, it's an optimal choice given a bad situation.

If you are relatively uncertain that any one child will survive and prosper, and you want to maximize the chance of having grandchildren, you have many children — which means starting early, too. Likewise, if you have limited access to health care, and you want to bear children at the time that it's medically safest for you to do so, late teenage years are preferable over early thirties.

Oh — and if it worked for your own parents (and those of others around you), it's evidently an effective strategy.

(We tend to overestimate the degree to which people's actions are due to the kind of person they are, and underestimate the degree to which they're due to their situation. To correct for this, when we see people in a particular situation behaving in a particular way, we should try explaining the behavior with the situation before introducing third causes such as that they are unusually stupid or short-sighted people.)