[skosch]

Your average household uses somewhere between 30 and 60 gallons of hot water a day [0], and a bigger tank will just have water sitting in it, going cold. Any extra surface area costs you extra heat, even if the marginal loss decreases with growing tank volumes.

[0] http://www.fsec.ucf.edu/en/publications/pdf/FSEC-PF-464-15.p...

[Xylakant]

You can use warm water to power your heating in winter. So it might be less efficient in summer, but the amount of energy you’d save in winter would exceed the losses in summer.