There's a specific benefit to cylindrical shapes, despite the increased surface area to volume ratio, which is the availability of vertical thermal stratification: water at the bottom of the tank is as low as ~2C, whilst at the top may be 50-60C (or higher). A diffuser inlet allows entering (warmed) water to settle at any level at which it is in equilibrium, without stirring. Water to be heated is drawn from the bottom of the tank. Thermal extraction through heat exchangers at the top.

The entire tank is packed within gobs of insulation. Cheap bulk insulation, rather than vacuum insulation, is effective, and additional volume accomplishes what the more expensive though thinner alternative delivers.