Conservation of angular momentum, if i remember my physics classses. Angular momentum is proportional to the rotating mass. and the distance away from the center of rotation. It requires energy to overcome the inertia of the wheels when starting from a dead stop; it also requires the same energy energy to slow the wheels down once they are rotating. This energy (to slow the bike down) comes from wind resistance, friction of wheels over pavement, internal friction of hubs, etc.
Any advantage of more massive rims is quite dependent on the number of start/stops and hills you have to climb. They would only help on a relatively flat course, say a TT course with no sprints or quick acceleration/deceleration, maintaining a fairly consistent speed.
Maybe a REAL engineer or physicist could chime in.