Why does diet soda fizz then reagular soda?

I believe that diet sodas are higher carbonated than regular sodas. The reason for this is that the artificial sweeteners in diet drinks have somewhat of a bitter aftertaste and they are "masked" by having the drink be more carbonated.

This also goes for high caffeinated sodas, like Pepsi Max is both diet and highly caffeinated, so I find it to be very fizzy!