Is that one of those tanks with a replaceable bladder that you need to stuff down through the bunghole at the top? I hear they are a nightmare to replace.
Yes that is a bag type bladder style tank. Customer requested it as it is fiberglass and this systems sits next to the ocean where corrosion is an issue. While bladder tanks don't last as long as diaphragm tanks, and fat diaphragm tanks last longer than tall ones because there is less stretch on the diaphragm, all types of tanks last a long time when used with a Cycle Stop Valve. The large system pictured was installed in 2004 and is still using the original tank.
On a large three pump system like that the CSV on the small pump keeps the system pressure at a constant 60 PSI nearly all the time, and the bladder in the tank does not move. When more water is used the pressure drops to 55 PSI and the CSV on the second pump maintains 55 PSI constant, and the bladder in the tank does not move. At 50 PSI the third pump comes on and the CSV maintains 50 PSI constant, and the bladder doesn't move. When people stop using water the pressure goes back up to 60 PSI and stays there for hours. The bladder in the tank contracts and expands slowly only 3-4 times a day, so even bladder style tanks last a long time.
A CSV holds the pressure constant while people are using water. So, the size of the tank is almost a moot point as the bladder or diaphragm doesn't move much and the tank can be very small as it is not doing anything.
Water comes from the well, cistern, pond, or other source, not the pressure tank. The only job of the pressure tank is to limit the number of pump cycles. When you have a Cycle Stop Valve to do that for you, a large pressure tank is a waste of money and space.