What? Do you have an example? That's not exactly an obvious statement and needs some clarifiaction. For example, Google says the Mazda 6 does 37-53 mpg combined and the Toyota Yaris does 58-67 combined. Your right foot must be considerably smaller and lighter than your left foot (assuming RHD) to be able to beat small cars in a much larger one.Most small cars actually MUCH worse than my previous Mazda 6 was with a very large (2.5 litre, 141 kW) engine and high weight (2t) consumed considerably less than many smaller and lighter cars.
Also, while I agree some politicians talk about reducing emissions when the measure will have only a small real-world impact, the real reason for cutting speed limits to 20mph is to reduce pedestrian (particularly children) injuries and fatalities, e.g. here:
Reducing the speed limit to 20 mph in urban areas