Without having the specific alloying percentages handy, it's safe to say that at LEAST 80-95% of the mass of ANY steel is still going to be Iron (Fe). So identical blocks of any two steels are likely to be about the same weight, because they are mostly made of the same stuff. However, assuming what the other guys are saying that SS is much weaker, then the SS bike would have to be beefed up to be as strong as other bikes, which yes will probably mean it's heavy.