We don't get unrequested fulls that often, and I would say we get them less often than we used to. The reason is obvious: the cost of mail, especially media mail, has gone up considerably. Yet some dorks remain who think sending their entire 600-page manuscript overnight (with return postage) is going to impress us somehow and not be a massive waste of money.
I don't know why this is, but unrequested manuscripts are almost universally awful. It doesn't make any statistical sense, but I can't think of one that I even considered putting in the maybe pile. One arrived today. To cut down on expenses, perhaps, the author printed out the manuscript single-space and in 8-point font, only to up the weight by putting it in a really big, nice binder (which we kept because there was no return postage and she said we could keep it). The writing was terrible - just terrible. You know it's bad when a chapter is 3 paragraphs long and two of those paragraphs are single lines of dialogue.
Maybe it's a jinx or something. I don't know.