To understand this you have to have a proper grasp of how aircon works.
In most basic terms the compressor is dumb and will compress the gas on demand, the cleaver thing about aircon is that it's actually the gas itself that decides how the system works and this is dictated by the ambient air temperature.
If you compress a gas it gets hot, expand a gas it gets cold. So in air conditioning the compressor squeezes the gas and it moves along to the condenser, where the now hot compressed gas is cooled, on a hot day it is harder to cool the hot gas, on a cold day it is much easier and efficient to cool the hot gas, is this is the fist step in understanding why aircon is very efficient in the winter.
The gas travels around the system where it will meet the expansion valve on expanding the gas gets cold and this is how the air con works, the exchange of heat here is the next step in the process of efficiency. At the evaporator, If the air temperature is hot then more of that heat gets transferred to the gas, if the air is cold then less transfer of heat takes place. Heat = energy so the more heat transferred into the system the more energy the air conditioning has to dissipate on the next round.
So the gas with more heat goes back to the compressor, the extra heat carried makes it harder to compress the gas, the gas gets hot again and then cooled in the condenser, now the condenser has to shift more heat, from the process and so the cycle continues.
In cold weather the gas does gather as much energy from the evaporator, it dissipates more energy at the condenser and the compressor does not have to work as hard to compress the gas, in an aircon system the resistance on the compressor, is the relative pressure of the gas and so the colder the weather the more efficient and less energy needed from the engine to run the system.
The compressor is controlled by pressure sensors in the system which cut the compressor if the pressure gets too high or low at various points.
If the weather is hot and there is a lot of energy in the gas then the system pressure will rise and so the system will cut out earlier.
As the pressure drops the aircon will start up again.
In cold weather there may not be enough energy going into the gas to keep the pressure high enough so again the system will cut out to let the gas move round the system and pressure at the low pressure side to come back up again.
A correctly gassed system is very efficient in this process, a system with too much or too little gas can cause big problems for mpg or power.
All aircon systems cut off at wide open throttle to allow more power to the engine. So if you demand the power it will be there.
In the summer the impact that aircon has on the efficiency of the engine and power, when the system is most demanding on resources, is still much less than driving around with windows open and the aerodynamic drag this causes at above 30mph if you're stopped in traffic just turn off the aircon and open a window, except you'll lose all that nice cool air the aircon made, so if you have stop start let the car turn off and on as it sees fit to maintain the status quo.
Over a prolonged period of time you might see a couple of MPG difference with aircon on rather than off, but if opening windows you will see s more dramatic change.
In truth carrying an extra passenger or a load of unnecessary junk in the boot will have more impact than using the aircon.
Hopefully this all makes sense