Cooling is one reason. Bigger batteries increase the surface area to volume ratio: heat produced in the active part of the battery has to be lost through the ca

Cooling is one reason. Bigger batteries increase the surface area to volume ratio: heat produced in the active part of the battery has to be lost through the casing and into whatever cooling system you're system. Bigger have less surface area. You also have packing issues: cylindrical cells are basically optimal, because they distribute any internal pressure equally. If you play with something like the prismatic Prius batteries, then despite being larger you have far less options to pack them because you need to counter-pressure them horizontally or they swell and then fail.