It's a fact:
Your lungs are not just empty bags, but filled full with the tiny convolutions of the tubules bronchioles and alveoli. In fact, the lungs resemble thick sponges more than anything else. Why?
One of the big principles in the structure and function of biological organisms is the surface area to volume ratio. Any organism has chemical needs (food, oxygen, water, etc.) as well as a means of interfacing with the outside world to get at those needs. An organism is limited in size by the rate at which it can conduct this interface. In the case bacteria, it is the plasma membrane, which is permeable to water, certain gasses and certain ions and has the ability to take up nutrients and expel waste. Thus, a bacterium needs a certain amount of plasma membrane to "support" the needs of the volume it contains.
The problem with this setup is that surface area doesn't grow in proportion to volume. If you took a happy spherical bacterium and doubled its diameter, you would find that while the surface area increases by a factor of 4, the volume increases by a factor of 8. This is why there aren't 80kg bacteria: the surface area would never be able to conduct the chemical business required by the volume it contains.
But how is it that I (80kg) can respirate, exchange ions and gather nutrients if my ostensible surface area is so small?
My system of interface is the lungs, which because of their structure have a surface area far larger than might be expected. In fact, the average set of lungs has the surface area of a tennis court. The intestines, too, are highly convoluted at the microscopic level to allow nutrients to absorb into the blood at maximum rate. These adaptations are what allow large organisms to survive, and, in the case of we mammals, are what allow us to achieve a small enough surface are to volume ratio to be successfully endothermic.