He's refering to Left Ventricular Hypertrophy.......some people believe that extremely intense aerobic activity that forces the heart to work very hard, will result in hypertrophy of the heart (it is a muscle after all).
I have NEVER seen any evidence to support this phenomenon exclusively attributed to cardio...PERMANENTLY anyway.
However, I don't find to be such an illogical theory. Although the heart is comprised of a different matrix of muscles (smooth vs skeletal).....there is "potential" for growth in response to work load!!! You commonly see this with COPD patients (as well as other cardiovascular/lung diseases/disorders).
The difference???
In the athlete's heart LVH may constitute a physiological adaptation to pressure overload, which normalizes following discontinuation of strenuous physical activity. On the other hand, in particular in patients with hypertension, LVH denotes a serious prognosis in the course of hypertension. In these patients LVH should be regarded as a grave prognostic sign rather than an innocent compensatory phenomenon. There IS a distinction between physiologic and pathophysiologic LVH.