In the right auricle impure blood is coming from the whole body through the superior and inferior venacava is collecting whereas in left auricle only the purified blood from the lungs is collected. So, to collect large volume of blood coming from the whole body, the right auricle is larger than the left auricle.
Atrium is the upper chamber of heart and auricle is the conical muscular pouch that arise from each atrium. Auricle is also called atrial appendage. Their purpose is to increase the capacity of the atrium and also increase the volume of blood that it is able to contain.
The right auricle receive non-oxygenated blood from upper and lower venae cavae and small veins in the thorax and direct it to the right ventricle. While the left auricle receive oxygenated blood coming from lungs through pulmonary veins and direct it to the left ventricle.Right auricles wall are thinner whereas left auricle walls are thicker.