Question: why are 12v batteries different sizes?
Answer: It is the amount of energy available in the battery that can give out one ampere of current to flow for one hour. For example for a 12 Volt 4 Ah battery, if you discharge the battery at the rate of 4 ampere the battery will get discharged in 1 hour. ... The thing that makes the two batteries differ is the Ampere-hour (Ah).