Originally Posted by LAwor

If you think about it quickly then yes but then ask yourself the question would I become more fit ( increase my heart rate more by) lifting 50lbs 10 times or 500lbs once or 100lbs or 5 times? Fitness is built through repetition. So if I have a lighter bike I can "spin" the crank faster, complete more repetitions which will increase my heart rate which by definition would increase my fitness. That's what the "spin classes" at the gym are predicated on.