For example, a tv remote needing two AA batteries instead of four AAA batteries.
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Devices have both power and voltage requirements. Voltage is based on what the components need to function. It’s much easier if a chip needs 5v to just give it 5v than to convert. Power is based on the collective consumption. More power hungry components more power needed.
Batteries have two features, voltage and capacity. Voltage is the actual voltage one battery puts out. If you connect one battery to the end of another battery in a line you get double the voltage. This is usually why devices have more batteries, they need 6v instead of 3v (for normal 1.5v AA batts) power is how many amps a battery can put out for a given time. For example a 2000 milli amp hour battery (mAh can put put 2000 mA or 2a for an hour. But it could also do 20mAh for 100 hours. The bigger the battery size the longer it can run. This is the second reason for more batteries, a longer run time.
With a remote it only uses power when buttons are pressed and when they are pressed they only use a very small amount of power. But critically the little IR led might need more than 1.5v to work. So it gets two batteries, the capacity doesn’t much matter but bigger = longer run time so it’s usually a size constraint, bigger remotes get AA, smaller AAA, really small might use a coin cell. But basically it’s pretty much just voltage and capacity that impact it
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