So if you go to buy a new, standard, tank-type water heater, you discover that for any given gallon capacity they come in several different tank shapes, called “short”, “medium”, and “tall”. It seems the “tall” are marketed for use in narrow spaces, the “short” for use in low spaces (like a crawlspace), and the “medium” for, well, everything else.
The interesting thing, though, is that from an efficiency (passive heat-loss minimization) standpoint, there is an ideal cylinder shape– namely, the one with the smallest ratio of surface area to volume, which is when the diameter is the same as the height. From this standpoint, the shorty models are almost perfect, but the mediums are too twice too tall, and the “talls” are even worse. Based on this, it would be rational to buy a short one– except that, for some reason, the shorts and talls are both much more expensive than a medium (like, $50-100 more)! This makes no sense from a materials perspective, since the shorts use less steel/glass/insulation than a medium, and a medium less than a tall. So I suppose it must have to do with demand, volume of production, and manufacturing efficiency.