Hi,

Get things straight : a 125 lb rider versus a 200 lb rider same fitness.

The bigger rider has all the advantages if as fit as the smaller rider.

Basic assumption is weight relates to power output so the bigger
rider has more power and less frontal area / power than the
smaller rider, clearly obvious comparing kids and adults.

Bigger wins everywhere except for endurance, you don't see
many sprinters running marathons. AFAIK is actually quite
difficult to build up the bulk of endurance muscles, compared
to twitch, but if you do short sharp hills you too can heavily
muscled legs related to building up the twitch muscle fibres.

rgds, sreten..
.