I had a good friend who unexpectedly died of the condition about a year ago.. He was always a much stronger rider than I was, and had a much lower resting heart rate, which I take to mean that his heart was larger than mine. My layman's theory on the subject (and of course I realize that I don't know what I am talking about because I am not a doctor) is that some people are genetically predisposed to greatly increase their heart size with aerobic exercise, and some people are not so predisposed. And when an athlete has developed a large heart due to strenuous exercise when young, his heart has difficulty dealing with its excessive pumping capacity when he gets older and doesn't exercise as hard as he used to.