It's the same with electrical power. High voltage may hurt, but it's current that kills. Size does not determine voltage. In everyday life, AAA, AA, C, and D cells all have a voltage of 1.5 Volts. But size does matter as far as current is concerned.
Big batteries provide more current than little ones.
Thus a small radio may operate on 2 AAA batteries. A larger cassette player may need 4 C batteries and a large boom-box may need 8 D batteries. As the amount of current drawn by the device increases, so must the size of the batteries. When two identical cells are connected in series, the voltage doubles, but the current is about the same as for each cell. In effect this makes one big cell with the same voltage as the individual cells but with much more capacity to provide current.