That’s the thing about pressure. A pressure vessel doesn’t care if it’s holding back enough gas to equal five cubic yards at 1 atmosphere, or 5 million cubic yards. It only cares that it’s at 3 bars or five.

You’re right, a vessel the size of a battery array is stupid. Because it’s all surface area and no volume, compared to a vessel the size of an aircraft, or a small stadium. You’d build one of these for an area of town, and the “size” that matters for stationary centralized equipment is not the displacement but the volume of material to make up the skin, the valves, the generator, and the piping to connect it all.