Small size, but enormous range

I took the cat to the vet today for an extended stay and some tests. First was the trauma of tracking her down, putting her in the pet carrier, and leaving her at with the vet. But when I returned home and stepped in the door I was overwhelmed at how empty the house felt. I've noticed this before, but today the feeling was strong. It's just amazing that a seven pound, relatively docile cat can fill a two story house with a palpable energy and warmth. Where does that energy come from?