Khan-Maidana on the undercard of pac-mayweather is a bigger event than them being the main event. The whole world will be watching pac-mayweather and if they do well the whole world will know who they are.
But the fighters (Khan and Maidana) would earn less money than they would if they have their own show. Being an undercard fighter to Mayweather/Pacquiao, means you'll be giving up most of your purse to the headline fighers.