Green is Willpower; Yellow is Fear
My boss has this tendency of asking idle questions at which I can take a competent swing at the answer. These are usually minor inquiries about the details of modern life that most people shrug over and quickly forget. Unfortunately for me, I have a habit of remembering curious little details and, when I manifest any understanding of these trifling questions, the boss wants to know more. This generally lands me back at my desk, investing an hour in researching some, otherwise ignorable, detail of the modern world. I've now got a stack of these one-page summaries of useless knowledge and I figure I might as well make use of them beyond entertaining my colleagues. So, here we go
WHY DO HOUSEHOLD BATTERIES COME IN DIFFERENT SIZES?
Dry cell batteries, the kind that you load in most household electronics, from remote controls to children's toys, come in two general varieties. There are 1.5 volt batteries, which are the cylindrical ones denoted by letters, D, AA, etc and there are 6/9 volt batteries that are usually square.
With 1.5 volt batteries, the cylindrical ones, the size indicates the lifetime of the battery. Larger batteries last longer so for maximum performance, a manufacturer of a device shoots for the largest battery that is convenient for their machine. Also important is “plate area,” the size of the battery's contacts. Larger plate area allows for greater amperage at the same voltage so wider batteries are needed to power higher amp devices.
Square batteries have similar concerns regarding size vs battery-life but are designed differently. 1.5 volt batteries have only one cell. 9 volt style batteries have higher voltages because they have multiple cells sandwiched together. They are shaped differently in part because it's easier to stack square cells together than round ones, in part to differentiate them from their lower voltage counterparts and in part to make them less susceptible to damage as they are more potentially dangerous than their cylindrical counterparts.
As for why one device would require six AAA batteries as opposed to two AA batteries, it's a matter of how batteries are wired. Batteries wired in series: positive to negative to positive etc, create a multi-cell battery with the same voltage, but a longer life, functionally the same as a larger battery. Batteries wired in parallel, positive to positive to positive then negative to negative to negative, create a higher voltage battery, with the same lifetime as one of the constituent batteries. A device may use a larger number of smaller batteries rather than a small number of larger batteries to take best advantage of amperage / voltage / lifetime wiring combinations.
Other, exotic types, of batteries like hearing aid batteries and those weird 6 volt cylindrical batteries used in light meters, are usually designed to conform to the needs of a specific device, hence why there are so many types of them. Because they are manufactured in such small quantities for very specific machines, there is little incentive to standardize them.
You are thus enlightened.