I am a newbie on using qgis server too. I also faced this problem and a solution is very easy but doesn't mention on official docs. You need to specify bbox parameter on url.
For the mentioned world.qgs project try BBOX=-25345974.386137474%2C16185170.55818786%2C34789346.44239271%2C23844254.527065746:
BBOX=-25345974.386137474%2C16185170.55818786%2C34789346.44239271%2C23844254.527065746