As far as I know, the intensive cardio workouts over a long period of time cause heart growth. The heart muscle is bigger and can pump more blood with one heart beat, allowing the same effort with lower HR (and much more intensive effort with the maximal HR).
Can I make estimation if my heart muscle has grown, comparing the same segments with the similar tempo over the long time period? If my HR is lower with the same effort, does it mean my heart is now bigger? Or there are much more factors to consider, like the optimized oxygen usage in the muscles?