This blog is about the sizes of portable power stations.
The size of the battery is about the power inside. Most batteries will feature a number on the front that tells you the number of mAh or “milliamp-hours.”
An international standard IEC has imposed a standard size but in Anglo-Saxon countries used the references in letter (AA, C, D, etc.) in other countries using references letter and digit (AA, Lr20, 6Lr61)
Simple, because there are so many different types and sizes of devices that require power from batteries. Some devices are physically larger and thus can provide a larger amount of space for a larger battery depending on current demands and how long the battery must power the device between charges. Other devices scale down to the tiniest of things like a hearing aid or the fit watch on your arm. These types of devices draw tiny amounts of electricity but still must run for the longest amount of time possible between battery charges. Add to that the fact that the battery must have a shape and size that is compatible with the device that it is being used in, and that adds even more different models in the marketplace.