I am far from an expert, but here is what my research has led me to believe...
Big brakes can serve 2 functions:
1) more braking force
2) Better cooling
So the question is, do you need either of these things and the answer is that depends. For #1, determine if you can lock up your brakes. If you can, you know that you already have enough braking force. For #2, you look at brake fade. You shouldn't experience brake fade on the street... this will apply more to the track. If you experience fade on the track, you need better cooling to reduce the fade and larger brakes will help you.
Now it is time for an expert to point out where I am wrong.