EFI is part of it, but also, a scoot with a bigger engine uses considerably less throttle to run the same speed that our Super 8 150's do at full chat.
I don't religiously track my MPG, but I have noticed that when I'm riding in town at 35-45 mph all day, which is a 150's comfort zone, I have easily beaten 70 mpg, and flirted with 80 once or twice.
If running on 55mph roads, we're pretty much at WOT, so the gas disappears faster.
A bigger scoot can run 55 and have throttle to spare, so they don't drain the tank as bad.
Duke-