More than you think. The lighter bike is likely to have much lighter wheels which physics says requires less effort to move from a stop plus the components like bearings and hubs are going to be better so the frictional losses from these components are less. I sometimes ride with kids who own cheap bikes. They can be pedaling down a hill and I coasting alongside and I will pass them up even when the rider+bike weight is closely matched. It is the combination of narrower tires and better wheels that makes that possible.