Because the heart is a pumping system, and it has no reason to be symmetrical, only effective.
-- The left side of the hear pumps the blood through your body, therefore it needs a lot of muscle power, so it's a thick layer of heart there. Right halve only collects the blood. So it will need a lot less muscled. I hope this answer was sufficient.
And, on top of that, the bottom half of the heart pumps blood to the entire body (which would be a lot of work), and the top half pumps blood to the head (which requires a much smaller force).