You can clean any size place yourself (I did a 7.5 room house) if you have the time and energy.
Cleaning companies send a crew, 3 - 4 persons, and they bring industial strength products along with tools and machines that make the task easier. They have a system, usually they divide the group, one does the kitchen, one does the bath, for example.
The place should be spotless when they are done. They remove things where hidden dust collects (light switch covers, light fixture globes, for example).