your kei car will do wonders if you drive it at 1200 rpm - which means 40 km/h in top gear all day long. These cars were not meant for "super fuel efficiency - they are meant to cater to a specific tax bracket.
let me give you a simple example.
I have a 3.2 litre engine car - the car weighs a lot more than your kei - by your rationale it should consume twice the fuel of a 1.6 litre engine, (sola dunee batees) - while infact it does not. converting to km/lit I get about 10.5 km/lit (25 mpg)
odd isnt it?