The main difference between CR2 and CR123 batteries is their size and capacity. The size of the CR2 battery is small and short as compared to the CR123 battery. Typically a CR2 battery  has 15.1mm dimensions while CR123 has 16MM long. Likewise, CR2 has a low capacity of about 500 to 800 mAH. CR123 on the other hand has a large capacity and can easily deliver performance within the range of about 700-1700mAH.