The video I posted earlier explains the "overpopulation/we are having too many children" myth very nicely.
People are already having less and less and less children. The biggest predictor of number of children is child mortality rates, access to contraception and education for women - and as this increases (as it is likely to) even in low income countries the birth rate reduces. This is already happening. We don't need to lower birth rates or have fewer children. It seems like the population is exploding and will continue to increase exponentially, but if you watch that's not actually what's happening.
The population growth is already set in motion. It's no longer stoppable - what we're seeing is adults growing up who never existed in previous generations, not because they died young but because they were never born in the first place. It's hard to explain in words but really, really easy to understand when Hans Rosling presents it. The relevant section is from 19:15 until 26:15, so it's not long. (It's very entertaining so you may find yourself watching more!)
Honestly I've never seen it explained ever anywhere else but it makes perfect sense and if it's true - which seems likely, looking at the UN population estimates - there's no point in trying to control population growth. The population won't increase indefinitely and we have already done what we need to do to prevent that from happening. Instead what we must work out is what to do with all the 11 billion people when they exist and basically that means accepting that we in the West aren't going to be the dominant global players any more.
Even the "top heavy" idea is a bit of a misunderstanding - we've always had fewer old people than other age groups because they died earlier and because previous generations have been smaller, but in the future it will be more even, which is not a situation we've ever had before, but doesn't mean the population is top heavy.
The frustrating part is seeing people get stuck on "we need to stop population growth" (we don't, and also, almost impossible) and nobody seems to want to talk about the two problems that are definitely going to happen even if everyone stopped having babies (which is unlikely to happen) - the numbers of older people and the numbers of people in general and the available land mass.
Another link in case you don't know what video I'm talking about and don't want to scroll back BTW