Is the world really overpopulated?

Everywhere I go, people are always talking about how we are overpopulated... there are too many people and there will be a crisis in 20, 30, or even 50 years. But are we really?

If that were true, then why are countries like Japan, Canada, Singapore and EVEN China* encouraging their citizens to have more kids? Where in the world are we overpopulated? and if so... why are countries more worried about not having a big enough population?... (I was even told by locals that a lot of third world countries are not able to keep their population either.)

These are questions... feel free to answer them in the comments below...

*China has recently cancelled their one child per couple policy and constantly encouraging their citizens to get married and have kids.