Why is a 6 volt battery larger than a 9 volt battery?

Answer 1

Nine volt batteries are actually 6 AAAA batteries. They have a higher voltage but a very low current capacity.

6V batteries are usually used in lanterns. As such they are a modest voltage but can source a fiar amount of current for a long period of time. This is why they are larger.

9V batteries can supply higher voltage, but the larger 6V batteries can supply much more power.

Another AnswerThe word "battery" means a collection of cells, where each cell generates a voltage according to its particular chemistry. To get more voltage, you put more cells in series. So, for example, your ordinary two-cell flashlight runs on 3 volts made from two 1.5v cells. A 9-volt radio can run on 6 1.5 volt cells. A 12v car battery is actually made from six 2-volt lead/acid cells, or similar.

The size of cells determines how long the chemical reaction will generate the proper voltage: the larger the cell, the longer it lasts. So you have standard sizes of AAA (short life), AA (slightly longer), C (much longer) and D (longest); all 1.5 volts.

When purchasing special-purpose cells ("batteries") where you have a choice, you may want to also compare the "ampere-hour" numbers, or milliamp hours (mAh), where the larger the number the more power you will have.