How do the walls of the atria compare with the walls of the ventricles and why are they different

The width of the muscular wall (myocardium) in the ventricles is greater than the width of the atria due to the force exerted by the heart to initiate major circulation (left ventricle) and minor or pulmonary circulation (right ventricle).

This is because in the human circulatory system, the major / systematic circulation that sends oxygen-rich blood (O2) and nutrients to the organs, starts from the left ventricle and ends in the right atrium, where blood is collected with the debris from cellular respiration. Then it passes through the tricuspid valve to the right ventricle and the minor / pulmonary circulation begins at the moment that the ventricle contracts strongly and expels the blood into the lungs, where the gas exchange in the pulmonary alveoli will occur again oxygenating the blood and being collected by the left atrium that connects to the left ventricle through the bicuspid or mitral valve and the process of major circulation begins again.