Any aircon will put more strain on your engine and make it work harder. Even in big engines this will be the case, though the actual physical difference wont be as noticable to you as the driver, it will show in your fuel consumption.
I have read about cars with electric aircons, but I am sure they too will use more fuel as the power must still come from the engine, be it with a belt that turns the compressor, or electricity from the alternator. I can just imagine that with electric aircons the effect of what it has on the engine power response that you might notice as the driver will be less. Perhaps I am completely wrong.
If you are on the highway in the city or on the open road to nowhere, then the fuel consumption increase due to the aircon should be less as the car has picked up momentum already and stays at a relatively constant speed compared to in the city where you constantly have to stop and pull away again. That is why they say rather use your aircon when on the highway compared to driving with windows open as the drag caused by open windows could actually have a bigger impact on fuel consumption compared to a aircon that is happily chugging along with a engine that is cruising at highway speeds.