Compare/contrast this question: electric car batteries are just big volumetric arrays of 18650 cells. If you know you need that much wattage — and you're manufacturing them yourself anyway — then why not make bigger cells, to reduce per-cell fixed-cost overheads and inefficiencies?
I assume there is a good answer to "why not" here; and it's one that's probably related to the "why" for SMR.
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.
I assume there is a good answer to "why not" here; and it's one that's probably related to the "why" for SMR.