Originally Posted by
Frank S
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.
Heavier rims/tires mean more inertia and it does require more force/torque to accelerate them, but that energy is not lost - it's stored in the wheel and assists the rider in between strokes.
There may be some efficiency difference in the biomechanics, like how quickly it tires out the muscles to accelerate slowly on each pedal stroke, or perhaps differences in the flexing of the tire (and frictional losses from that, if the tire flexes more during more of the downstroke, like especially if the rider is out of the saddle), but in terms of rotation of the wheel itself, there is probably relatively little energy lost regardless of wheel weight or weight distribution.