There are already electric cars that use different cell technology (such as prismatic cells). Larger cell sizes are a tradeoff, on the one hand the amount of packaging overhead per mAh decreases (it only takes one container for a 100Ah cell instead of 30 cans for 30 cylindrical 3.3Ah cells); on the other hand there are several disadvantages:
1. In a crash, a pack with large prismatic cells will have the impact will rupture and short-circuit one or several ~100Ah modules - whereas with the Tesla-style individual cells, the same crash will only take out the handful of cells closest to the crash direction site.
2. Defective cells, say those that develop open or a short circuit failures, will have a significant effect on the performance of the pack. In a Tesla-style pack with thousands of cylindrical cells, one shorted cell will pop the fuse for that cell, and the pack continues to operate with marginally decreased capacity
3. Larger cells make thermal management more difficult. Smaller cells are more inefficient in terms of the ratio of active cathode/anode material to packaging material (the metal case, contacts, safety features etc.). Somewhere between the two extremes is a sweet spot. Tesla's studies determined that 20700 (and later 21700) is that sweet spot. It certainly looks compelling. A 21700 isn't that much larger than a 18650, but packs 50% more capacity.
4. Larger cells are more demanding from a production standpoint. In much the same way that manufacturing a huge TFT screen is much more difficult than manufacturing a small one (you can only have 3 dead pixels, and there are more pixels in a 52" TV than in a 21" monitor), with larger batteries you need greater areas of electrode, so the probability of defects getting into the batteries is greater (defects meaning small regions of anode/cathode/seperator material that are on the edge of acceptable tolerances). In a small cylindrical cell these are easily weeded out at testing because they really affect cell characteristics; in a 100Ah prismatic cell the same defects will probably go un-noticed at production, then 5 years later you end up with a hole in the separator and that cell develops high self discharge...
I think 21700 is here to stay for at least another decade.
