left side of heart

Cause the right side is the part pumping venous blood to the lungs.

It gets the blood from the rest of your body blood that is pumping

one side piston is broken

Here is a nicer answer to your question:

Because if you think about it, the left side of your heart is pumping blood to your ENTIRE body, while the right side is pumping only to the lungs. for example, say if you used your left arm to lift weights twice as often as you did your right arm, your left arm will naturally be larger cause it works harder. Hope this answer your question.