It's physics. Push a heavier thing - you need more force; you need to do more "work" as in the physics kind. WOrk takes calories. More work takes more calories.

Of course, it's not a linear relationship and the fitter you are the less difference it makes! I burned extra calories today on my big red bike toting the cooler with the ice and bananas and yogurt and the blender apparatus... and didn't have *too* much smoothie to undo it

I'm afraid I'm just plain addicted... and right now tired enough so that even though the next ILlinois rider is a big fat four miles ahead of me I have *no* intention of taking th3 5 mile route home Let 'em go... my rides are **my** rides. (And when I *am* feeling competitive, I tell that side that welp, none of the folks ahead of me are on 40 pound hybrids most of the time )