McDonalds Employees: Why is the McFlurry so much more fattening than the sundaes?

The plain sundaes are like, around 150-300 calories, So when I got the McFlurry tonight, I figured I was only adding maybe another 100 calories in oreos...WRONG... it was 580 CALS!!! WTF? Does anyone know why? Is the mix different? If you work there, I'd like an explanation..thanks!