system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: You can use AAA batteries for electronics that require AA batteries, as long as you fill the space with something that conducts the power. So why does both battery types need to exist? Why not just have one size fits all?
gpt
Battery capacity - Because AA batteries have more battery capacity than a AAA battery. Filling the empty space with some conductive material with AAA batteries would mean this device will run out of juice faster.

Device size - If a device is small, it may be not be practical to allow a AA battery as opposed to a AAA.