A 100 kg rider has double the kinetic energy of a 50 kg rider when both are traveling at the same speed. So if the increase in drag (aero + rolling resistance) doesn’t outweigh the difference in kinetic energy, the heavier rider should have to apply less W/kg to maintain the speed than the lighter rider. If they apply the same W/kg the heavier riders will go faster. This is my understanding of physics involved, but I might be wrong We can ignore wind, air density and other factors that are the same for all riders.
Quite simply, kinetic energy is the energy of motion. In this case, the energy contained in you — and your bike — when you’re already in motion. The measurement units for energy (either potential or released) are joules. Power is expressed in watts, which is a joule per second.
So if I’m 80kg and I’m riding at 20km/h, my kinetic energy will be roughly 1,230 joules (Ek = ½ x 80kg x 5.55m/s2, where 5.5m/s is the same as 20km/h).
If I’m doing 50km/h, my kinetic energy will be nearly seven times higher — 7,720 joules — because as mentioned in the equation, velocity needs to be squared.
If you’re travelling 50km/hr on a flat road and you stop pedalling, you’re still moving quickly and won’t slow down very much. You’ve got all this energy which helps to overcome the drag and rolling resistance. So you’ve got a lot kinetic energy in the system, but the forces that are retarding you are relatively minor, so you don’t slow down very quickly.
Reference: The Physics of Moving a Bike - Slowtwitch.com