Bladder tanks have vinyl tanks that contain water from all sides. Diaphragm tanks have flat diaphragms mounted two-thirds of the way from the top of the tank.
A seam on the outside of a steel water tank is often an indication of the presence of a diaphragm. Both bladders and diaphragms serve the same function, creating a pressure differential using compressed air that pushes against the water, ejecting it from the tank. Bladder tanks are more compact than their diaphragm counterparts because pressurized air pushes up against the bladder from all sides, whereas this pressure pushes down from only the top of a diaphragm tank.
The bigger a pressure tank is, the larger the amount of water that can be stored under pressure. Using too small a tank with a pump can cause the pump overcompensate for the lack of pressure when the tanks empties, leading to premature pump failure. By keeping the pump from having to start often to fill a smaller tank, the contacts of the pump motor can be preserved, prolonging the pump life. A larger tank enables the pump to economize, filling the whole tank in one prolonged session and then allowing the tank to empty over time.