Generally speaking, this is a "problem" that fixes itself. Rising prosperity and economic development tend to result in lower birth rates. This is why the One Child Policy in China was arguably unnecessary, as well as unarguably brutal and inhumane.
However, even if everyone in the world had the same birth-rate as the members of the OECD, there's no escaping the problem of numbers when it comes to sustainability. According to the World Bank development indicators published in 2008, 80% of the human population lives on less than US$10 per day. Just how low-resource a life-style will they and their 2.1 kids have to adopt to be sustainable? Never mind SUVs, or imagining everyone in India living the American dream; just providing everybody with bicycles and refrigerators would maintain a pretty massive demand for resources. I don't think anyone in the developed world is remotely ready to confront just how low a level of resource consumption would be involved in an
equitable, sustainable world.
Arguably even the relatively low standard of living (by rich-country standards) enjoyed by the citizens of Mumbai or Shanghai is unsustainable. At purchasing-power-parity (PPP), the average global GDP per head in US$ is around $11,000 per year, or a little less that of Brazil and less than a quarter of that of the USA. Yet most of the human population lives in countries that are poorer:

Blue is above the Global Average PPP GDB. Orange is below.