Think about it logically. The Coronary ciculation only goes arount the heart, a fairly small part of the body. The pulmonary only goes around the lungs, again a relatively small percentage of the body. The general systemic circulation has to reeach every where in the body and get back again, that means the whole body making this the biggest. The lymphatic circulation, while it does have to come from everywhere in the body it only has to carry fluid back in one direction and so is only half the size of the general systemic circulation.