North and south Sudan have suffered decades of conflict driven by religious and ethnic divides. Southern Sudan is one of the least developed areas in the world and many of its people have have long complained of mistreatment at the hands of the Khartoum government.
There are an estimated 3.8 million registered voters in the south of Sudan, out of a total population in the entire country of Sudan of about 44 million. Thus, the south is considerably smaller demographically than the north. This is exemplified by the size of the capital of Sudan, Khartoum, which has a population of 5 million, compared to the population of Juba, the new capital of the new country (which has yet to be named), whose population is estimated to be no more than 250,000--a number that I have drawn from this intriguing (albeit not well documented) entry from Wikipedia:
In 2005 its population was 163,442. Based on analysis of aerial photos, the best estimate of several donors working in Juba calculated the 2006 population at approximately 250,000. The 5th Sudan Population and Housing Census took place in April/May 2008 but the results were rejected by the government of Southern Sudan. Juba is one of the fastest-growing cities in the world, and is developing very rapidly due to oil money and the Chinese coming for work and development.