What I have learnt from my mechanic and personal experiences, following factors are the main reasons in making Suzuki bikes fuel efficient.
1. Butterfly carburetor: Seen on the internet as well as per the experience, I have found this type of carb more fuel efficient. Mainly due to indirect throttle i.e. rider twist the butterfly which indirectly opens up the throttle needle (as per need).
2. Battery involvement: I have seen in practical in GS-150 that if its battery is weak then it consumes more fuel. My cousin experienced this as well, he was getting 45km/lit which suddenly drops to 20km/lit. He did nothing else just replaced the faulty battery with good one and his life returned to peace.
3. Piston diameter and valves: Off course piston and valves are major factors. What I have learnt that smaller diameter piston and smaller valves are key to fuel efficiency. And the Suzuki knows this very well (as I am talking only about Suzuki here so I am not saying others have no idea about this, peace ).
4. Head - Smaller sized mixture intake (hole) and exhaust. From one of senior mechanic at a renown service center, I have come to know that Sprinter head has smaller than normal intake and exhaust hole which is helpful in creating more pressure inside the cylinder thus consuming the air-fuel mixture more efficiently.
5. Little wider spark plug gape: In that very initial period when I newly got my Sprinter, I use to keep the gape same as of 70cc. Luckily I blessed with a specialist of Suzuki's and he tuned my bike and widened the plug gape. I will try to get a snap for you guys. He tuned the carb as well and other tweaks like valve adjustment and there you go. I felt great improvements like lighter ride, mileage increased from 60 - 63 km/lit to 65 -68 km/lit.
Later on (for no reason ) I again narrowed the gape and immediately felt heavy ride so afterwards I stuck to wider gape and it has been 2 years now.
Other factors are obvious like overall bike weight, rim, tyre size, sprockets ...etc.