Its in the Gameplay Constants. There is a mob exp table and it multiplied by the lvl of the mob. I honestly dont fully understand how it works but I was able to change the exp given for killing a mob. Killing a lvl 1 mob gave me 2 exp, lvl 2 gave 4, lvl 3 gave 6... and so on. There should be a way to set a specific mob to give a desired amount of exp but unfortunately I dont know how to do that and if anyone finds out... please let me know too. I wish there was a detailed faq on the Editor and Gameplay Constants so I knew what everything did. Good luck and let me know if you need further information.
Dethlor