If you are a hot water tank user there is one thing that you cannot help but be impressed with is that tankless hot water tanks are really small (if tanks is even the right word, maybe box would be better?).
On demand water heaters do not need to store water, so there is absolutely no need to have a thing big enough to take 80 gallons of hot water - it is a simple as that.
Of course tankless hot water heaters have a fairly impressive amount of work to do to raise the temperature of the incoming mains water to a good hot bathing temperature close to instantly.
This is why a tankless gas water heater (for example) still is a reasonable size - it needs room for the heat exchanger, which is where the water is spread out and heated up by the gas flames.
Though seldom as powerful as a gas counterpart, an electric tankless water heater can be even more impressive, size-wise. These units can be happily hidden away behind a bath panel or unobtrusively at the side of the dishwasher.
When it comes to a battle of size and you compare tankless water heater with a tank equivalent, there really is no competition. Tankless every time.