They are related to some degree. You want the sump big enough to allow for the equipment needed as well as enough room to handle water in the pipes (power to return pumps cut) in addition to air bubbles to diffuse. With a bigger tank, you will be running a bigger pump- thereby when the water returns down the pipe, it will be making lots more bubbles. Many people also want their sump to fit underneith their stand, so that factors in, and if external pumps are used, that impacts the size that can fit as well. Most people opt for as big as practical.
