I just looked it up because I wasn't all that sure.
White pepper comes from the same peppercorn as black, but it consists of the inside of the peppercorn only. From what I read, it's flavor is "longer and drawn out, tending to linger." I think that is a great way to describe it. Maybe it isn't that much hotter, but it certainly seems to linger, and I think it "numbs" your mouth more.
But then again, it might just have to do with the freshness. Fresh pepper, of any kind, seems much bolder and stronger.
I buy the black pepper in the bigger jugs and I'm sure it's not all that fresh. I can only find the white pepper in small containers and we go through it pretty quickly. That may likely have more to do with it than anything.