Big Dog, Little Dog
Slate reports that America's most popular dog is the Labrador retriever. Slate posits that the reason for this is likely that the average size of an American home is increasing, so therefore the size of the average American dog must also increase. In other words, if you supersize your house, you must therefore also supersize your dog.
I think I prefer James Herriot's view of people and their dogs. In his experience, wealthy people with enormous English country houses had tiny dogs. And cottagers living in tiny one room abodes had a large dog, often more than one. I like the incongruity of that scenario better.