In my experience it's a time/volume tradeoff. The longer it takes, the higher the magnitude. It's funny because Benoit Mandelbrot talked about this in The (Mis)Behavior of Markets and seemed to indicate there was some algorithm behind these sorts of "repeats". His theories are so anti-modern finance though, that most people ignored what he had to say.
That doesn't mean enterprising Bitcoin traders aren't taking advantage of it or evolving on what he started, though.
Interesting... I didn't know about that. At the beginning of this thread I likened the behavior to fractals we see in nature, so maybe this is indicative of the trend in finance (at least when artificial limitations have not been placed on markets).
Apple is a great company but it can't keep growing at the rate that it did when it was two guys in a garage, right? The market cap is too big.
I think this is why we are observing a smaller percentage change each cycle. It is indeed harder to change something that is already getting large. Of course, Bitcoin still has a large potential future ahead of it!