Atria and ventricles differ in size, location and method of operation, according to Wikipedia. These differences determine the thickness of walls and the type of blood vessels connecting to each type of chamber.
The shape of the heart is roughly analogous to that of an upside-down pear, notes the Franklin Institute. The two atria lie at the thicker side of the heart, while ventricles form the narrower end. Atria receive blood from the body and eject it to the ventricles. Because of their less taxing role, the atria are smaller in size than ventricles, notes Wikipedia.
The right atrium receives deoxygenated blood from the body and ejects it to the right ventricle, according to WebMD. The right ventricle then pumps this blood through the pulmonary artery to the lungs for oxygenation. After oxygenation, blood flows through the pulmonary veins to the left atrium, from where it is ejected to the left ventricle. The left ventricle pumps the oxygen-rich blood to the rest of the body.
In general, ventricles need to generate greater force and higher pressures. For this reason, ventricles have thicker walls and more powerful muscles than atria, Wikipedia explains. Because the left ventricle needs to generate enough force to pump blood throughout the whole body, it has the most powerful muscles of the four heart chambers.