For you to be able to get the population density you need to divide the population by the size of the area. The formula looks like this:
Population Density = Number of People/Land Area
= 683,000/126 square miles
= 5,420 people/square miles
If it grows to 1,000,000 people the solution is:
Population density = 1M/683,000 square miles
= 1.46 people/square miles