Originally Posted by Pedaleur

For the record, on the flats, inertia and momentum aren't what's important. For a constant speed, acceleration is zero (by definition), and so you merely have to balance the propulsion and drag forces. The larger rider has more drag, but, as noted above, the larger rider tends to have even greater increase in power output.

The "larger rider has more momentum" rationale is incorrect because the ability of a heavy rider to "carry his momentum" is offset by the fact that it takes more energy to increase his speed through pedaling.

Up and down hills changes everything, of course, because changes in potential energy are proportional to mass.

Originally Posted by umd

Inertia is important relative to a gust of wind. It's not about momentum, it's about the wind's ability to move the rider...