>The three-car garage McMansion with double-height ceilings and a double-height foyer is a ridiculous amount of space to cool down and heat up for a single household.

That's definitely not a significant contributor to American energy usage, given that not everyone in America is a millionaire.

There are 139 homes for sale in the Dallas Metroplex that have 6 or more bedrooms for $600,000 or less.

There are 1.318 million Dallas residents. 139/1.31M = 0.0001. Not too significant.

That's not that amazing of a metric. For starters, households are what track to a house, not residents. Then you also have to consider what proportion of that population is looking to buy a house at any given time; most people tend to stay in a purchased home for a long time, so they may already live in a McMansion style house.

McMansions are McMansions because they are cheap to construct and cheap to buy (and surrounded by thousands of nearly identical structures).