When I had a 180 gallon tank, I ran a 250W heater and it was on only 30% of the time. I find large tanks retain heat better. Not sure if it is the thicker glass or the larger volume of water. I am thinking perhaps the larger volume of water. I am not sure glass is the greatest insulator.
I have heard the argument of the two-heater theory. I'm still up in the air about it. Two heaters are good so that if one quits you still get some heat. On the other hand, aren't you doubling the chance that one heater malfunctions to the on position and you fry your fish?
Hello; First the larger body holds heat better idea. In biology I sometimes referred to the inverse square rule. Roughly when the outside surface area of an object doubles then the internal volume increases by about four times. It is one reason why animals in cold climates tend to be larger of body.
Of course an aquarium is not alive so is not generating it's own heat from cellular combustion (oxidation). I do think the greater mass is the key. Water also is good at holding heat. Have you ever swam in a lake in the fall when the air temp is cold but the lake water is still warm?
On the two heater thing. I have had heaters fail both ways; stuck on and stuck off. With a single heater of sufficient wattage to maintain the tank temp that sticks on it can overheat a tank. So if 200 watts is just right to maintain the water temp and you have two 100 watt heaters then it reduces the chances of cooking a tank if one sticks on.
Same sort of idea if one fails. The other may keep the temp from getting too low in the meantime. I guess your notion of the chances being doubled for failure with two heaters is in some ways correct but not the risk to a tank. Anyway I use two or more heaters on larger tanks.