That's obvious. But it's not like it's going to make a lot of difference in making you stronger. My old mountain bike is 10 pounds heavier than my hybrid. Not a lot, really. For example, if you do weight training, 10 pounds isn't a huge difference and you should be able to add 10 lbs of weight within a couple of months but it won't be long before your body adapts and then you have to increase the weight. I think when it comes to getting a better workout cycling, ride faster and ride uphill -depending on the geography of your place of course- and ride faster going uphill. I'd rather have the option of being light (lighter bike). But if I want more weight I can always wear those exercise vests/clothing (not sure what they're called) that you can load with weight blocks. I always see guys wearing those.