Mithion said:
https://goo.gl/images/SBep9p
Why is the 18650 so popular say over the 26650 or any other size?
Is it a perfect size for the power thing?
Many reasons, but primarily its a price thing together with choice of supply.
18650s were so widely used in laptops from the '90s through to the last few years, that a hugh manufacturing capacity was established by all the big suppliers as well as other minor players.
As the laptops slimmed down and subsequently moved to custom made batteries, much of that 18650 production capacity went begging for an outlet, driving down prices and pushing up availability.
Power tools helped move the chemistry to the higher power and capacities , whilst retaining the proven 18650 manufacturing quality and leveraging volume costs.
Other formats ..26650, 32500, 38120, etc etc, have existed for many years, but they have never had the volume of production to reduce the cost of manufacturing in the scale of 18650s.
The cream topping was probably Tesla adopting the 18650 as its prefered format for several reasons (thermal dispersion, safety, cost, quality, availability etc) ..and then pushing development to new levels with Panasonic.