Look at the bottom right corner. It has the current population of the server represented by bar graph for each empire. In this picture, the VS has a 2% experience bonus and the NC has a 20% bonus. The TR has the most people at the time of this picture, so they do not receive an experience bonus. I think it also says "resources" under "experience." Guess underpopulated empires get a resource bonus in addition to experience.
In the original game, underpopulated empires also received an experience bonus for kills.
full size picture
http://a.yfrog.com/img532/5848/95rqd.jpg