Yes, mainly in the countries with the lowest birth rates. Take China for instance. Indeed, their huge population is much better off by standard measures, than it was a couple of generations ago. But look at the intense program to curb births that went along with that.
Make a map of the world with each country labelled for its birth rates, per capita income, and life expectancy, and I think you'll see that more babies does indeed translate into more misery.
You're confusing cause and effect. As life gets better for people, as income and life expectancy rises, after a brief lag time they begin to have fewer children.
Fewer children does not create a better life for people. The better life leads to fewer children.
This process has been observed for over a century as countries modernize. I don't know of a single exception to the rule. Except maybe Utah.