Well, because if your crank pulley is smaller, that will slow the rotation of ALL YOUR ACCESSORIES. Your engine won't have as must drag from the accessories saving some ponies (generally 5-15).
With kits, usually the water pump pulley is BIGGER making it slow even more. If the alternator pulley were bigger, it would slow more, too.
If you change just the crank pulley, you will definitely need a shorter accessory belt (I could find the exact one you will need, if you need that info).
