When the moisture in the air is high, it means that there is more heat contained in it. This makes the air conditioner have to work harder as on the one hand, it needs to remove all the humidity from the air and on the other, it also combat the warm air to the space.
On the contrary, in the winter days, the weather is colder, the air is also drier. This makes the heater bring into play all of its efforts to balance the moisture because too low humidity cannot hold the heat, which means your room will be cold and it is not good for your health.