Thus, the one main difference between these two batteries is the diameter. As you can see that 26650 battery is larger in diameter as compared to 18650 battery. 

2.Capacity
The general rule in lithium-ion batteries is a higher capacity for a bigger size.  the capacity of 18650 rechargeable Lithium-ion batteries is around 1200mAH - 3600mAh, while the 21700 battery ranges from 3000mAh - 5000 mAh. Meanwhile, larger batteries need more time charging time.

3.Volumetric Energy Density
This is a measurement of space efficiency and indicates how much energy a battery has in comparison to its volume. 18650 batteries have Volumetric Energy Density of around 610 Wh/L while 21700 batteries are in the 590 Wh/L range. 

4.Applications

18650 Lithium-ion Battery: 18650 Batteries are commonly used in Laptops, Electronic devices, UAVs, Flashlights and in electronic-cigarettes (Vaping). Some electric cars also use 18650 Lithium-ion battery pack. Tesla has routinely used 18650 battery pack of 7104 cells manufactured by Panasonic.