I'm sceptical that there are so many efficient bikes out there. My understanding was always that 2 stroke engines were not as efficient as 4 stroke engines.
They aren't*, and I'm extremely skeptical that a tuned 125cc two stroke could ever return 80mpg, especially with a pillion.
The small four stroke engines are very economic however, there are plenty of scooters that can manage 120mpg, and the Jap four stroke 125 bikes should be approaching 100mpg if they are in a decent state of tune (and you aren't flat out all the time).
Weight is a bit of a killer on small bikes simply because they are so light to start with; the difference between say 10 stone and 17.5 stone is more than a third of the weight of the bike
Also the type of riding you do will have a big influence; is your commute only mainly open roads with few traffic lights, or do you spend a lot of time in heavy traffic?
Low tyre pressures, binding brakes and insufficiently lubricated chains will all impact economy as well.
*Apart from supercharged two stroke diesels which are one of the most efficient internal combustion piston engines.