Like the other responses, the battery chemistry is the major reason for cell sizes, but also, smaller cells means they can be isolated if necessary.

In a Tesla battery pack for instance, each cell is connected by a single small wire that also doubles as a fuse. If there is an issue with the cell the wire will heat up and break. opening the circuit and separating that battery cell from the rest of the pack. This also means that a failed cell doesn’t take out a significant portion of battery capacity. Other manufacturers do this as well, but not all, and some implement similar capability in other ways. This method functions as a sort of “passive” option since it doesn’t require the BMS to make a decision to remove those bad batteries from the pack, physics just does it and the BMS adjusts to compensate when the cell no longer is connected.

Good battery management systems that handle things like charge leveling individual cells, can mean the difference between batteries degrading noticeably in a few years and the pack as a whole lasting a decade before that noticeable degradation. There are a lot of poor battery management systems on the market, EVs are no exception and if anything they make this issue more noticeable because of the increased usage.