Crank pulley is smaller, and to compensate for loss of power steering pump, alternator, etc. the accessory pullies are usually larger. They're all lighter though, because most underdrive stuff I've seen have been alloy instead of steel. One thing to be mindful of is the harmonic balancer. If it's built into your crank pulley and you put an underdrive pulley on that doesn't include one, you're going to cause more wear on the motor or at very least increase NVH. There's a reason it's on there from the factory, but if you're ok with it anything goes. I had one on my Subaru for ~45k or so with no known adverse effects. Maybe the engine will only make it to 150k instead of 200k before it needs work? Who knows, but that's completely fine by me.