Assuming that all else is equal, it's not necessarily the total weights of each wheelset that matters. What counts most is the locations of the weight. If the 700c wheels have lighter hubs, but heavier rims+tires, than the 650b bike, all of those little accelerations will take more force to accomplish. If the 700c rims+tires weigh exactly the same as the 650b, but have a larger diameter, those little accelerations will take more force. If the 700c rims+tires weigh more than the 650b, and they have a larger diameter, they'll need even more force.