What? The power bill is not insignificant, it is irrelevant when talking about battery capacity. The size of the battery has no bearing on the energy cost. If the device consumes 2500 mAh per day, that costs the same whether the batter is 10,000 mAh or 500 mAh.
The downsides of larger batteries are increased weight, volume, and cost. Being more efficient gets the same battery life for less of those three, allowing them to either be reduced or the budget to go to other components.