Why does non-regular soda fizz more?!

I've been flicking through the internet, and a lot of people seem to think there is no difference in fizz. Except, I work fast food, and whenever I have to make drinks that are not regular Coke (i.e. Coke Zero or Diet Coke), I end up getting huge amounts of froth for a teensy amount of coke, whereas the regular soda works with minimal fizz.

Any explanation other than "um, there's no difference" would be appreciated. I've seen it. It happens.