Once again, there is a perfectly good explanation for this, and it's all physics! The equations are extremely complexe, and I will spare you the details (but if you want them, you can download a paper by Princeton University student Kirk T. McDonald on the matter. Just read pages 2,3,4 and 5). The basic idea is that when you increase a strings diameter, you also increase the amount of magnetic material above the pickup. When this larger amount of material vibrates within the magnetic field, there is a greater disturbance than if the string was of a smaller diameter. Now, without explaining why in this article, the disturbance of the magnetic field in an electric guitar pickup is what generates the electric signal. And, the disturbance is directly proportional to the electric signal created.

So, now you know why Stevie Ray Vaughan sounded so awesome, even though I'm not crazy enough to play the electric blues on a strat with a set of 13's... 11's is plenty enough.