/end nitpicking rant

OP, I think this is really a gearing issue as Danni suggested. If you have sufficiently low gear(s) on your bike, the weight is inconsequential. Correct me if i'm wrong, but I think by "strength" you mean "torque" - putting a heavy load on each pedal stroke due to being overgeared.

And yes, you can absolutely increase your "strength" by riding a heavy bike with a big gear. Sorry to nitpick. It's just the gear thats making u strong (not the weight of the bike) is all im sayin'