The compressor does not run continuously. That is what the thermostat is for. When it is cold enough, the compressor stops. When it gets warm, the compressor starts again.

No insulation is perfect, so "cold" is lost to the outside. Looked at another way, heat is seeping in. The worse the insulation, the more heat seeps in, the more the compressor has to work, the more electricity is used.

In addition, the bodies of the people inside the room generates heat. More people also cause more electricity to be used.