From some posts on this thread it seems you can go to the gym and lift little weights, and get the same effect as the guys who lift the big ones.
You're analogy only works if by riding a lighter bike the rider is not working as hard. If you work to the same intensity you'll get the same results. You just have to go faster to do so on the light bike.
Turn down the incline on the treadmill, the resistance on the spin bike, makes no difference.The resistance on a bike isn't controlled, in the main, by it's weight.