Enjoy McMansion Hell - http://mcmansionhell.com/

I call BS on the single-story build rate > 40%. Finding my current home in 2015 - a single-story - took a lot longer because they weren't that many on the market at a given time. Some were under contract in less than one day on the market! But I am confusing build rate with market availability.

The blame will fall on HGTV for some of the problem.

Another factor is profitability - a 2000 sqft house costs less than 2x to build than a 1000 sqft house, but within the same hypothetical neighborhood, will probably cost 2x or more because of the additional features that additional floor space makes possible.