It's an interesting Physics problem acutally that's somewhat complex. A previous poster mentiones system CG which is accurate. You will raise the system CG and your CG when standing. But the system is actually 2 bodies with complex interactions. The forces applied from the body affect the motion of the bike. Standing distributes the loads to the freely moving body (bike) through the footpegs as 2 point forces. Drawing a free body diagram, this allows for the rider to interact with the bike in significantly different ways than putting a point load through the seat while sitting. I don't have the time to do the math right now, but transferring loads to the footpegs that is lower than the bikes independant CG will have implications to the control of the bike as opposed to merely shifting your weight while sitting. The way that inertia affects the system is an interesting problem as well that I will have to put some thought into.