OD*ID for 2ml is wrong. It actually is the same as for 5ml, 23*20. The 5ml is accomplished by the extender which makes the tank higher, not wider. The small glass is the chamber around the coil, it's not the tank.
Also under Dimensions, height 45mm is for 2ml setup (underside 510 to top of drip tip), with extender the height measures 54mm (54.4 actually).

Same for SKU 9711815.