World Travelers is different from the normal 
Citizen Population. It can only be increased by building World Travelers Residences. To allow more World Travelers in your city, you need to build World Travelers Community Buildings.
When expanding, your World Travelers will be combined with your normal Population.
Once the Discover The World Saga/Event ends, the World Travelers/Allowance will be added to your total population/allowance and World Travelers will change to regular citizens.