[RedDevil]

Actually, the idea behind using many small cells instead of a few big ones stems from the unreliability of lithium-ion cells.

Over time and use dendrites form between the cathode and anode and when they meet the cell shorts and dies. If you have a battery with big cells in series that will reduce the voltage of that pack if it is able to bridge that cell, or kill it entirely if it isn't.

Whereas the Tesla approach has 40+ small cells in parallel with a thin lead to each cell which acts as a fuse. If a cell fails it will get out of line with the other cells in its tier, draw more current and blow its fuse. It is then isolated and the tier remains at 98% of its original capacity. Even if dozens of cells fail distributed over the pack the total capacity only drops significantly if many of those happen to be in the same tier; usually the failures will be randomly spread over the pack and no tiers will have more than a pair or so. The pack is much more robust.

Advances in production technique have made lithium cells more reliable over time, so now it is no longer essential to split the pack into small cells. The Model 3 and Y already use bigger cells than the S and X and their packs seem much more robust nonetheless.