> Is it because they have lower expectations for what their children need to live a "good" life
Yes, I think the minimally acceptable quality of life is certainly relative.
> that they just have more hope that things will work out somehow
Possibly, if everyone around you is on an upward trajectory, I can see that changing people’s calculus.
> or that they somehow don't care about these concerns?
I think a big factor is how (financially) independent women are and what kind of access to birth control (especially IUDs) they have. I suspect many of the women who have or had 3+ children would not have if they had similar options to those in developed countries today.