In the 1950s, most new homes were around 1200 square feet and held about 4.5 people. Today, the typical new home is over 2400 square feet and holds about 3.5 people (IIRC).
I see zero reason why 2400 square foot homes would lose any value whatsoever if we built more small homes so people who can't afford a house that big can still afford a place of their own.
One factor why house prices are high is that demand exceeds supply. By building small homes, a lot of demand would be catered to by a new pool of housing. Heck, even those who could afford bigger houses but prefer smaller houses for whatever reasons (eg. frugality) would be removed from demand side. So now there would be a downward pressure on house prices.
This means that downward pressure on home prices amounts to slowing hyperinflation to a more reasonable rate. This isn't remotely the same as gutting the equity for existing homeowners (much less putting them "upside down," where they owe more than the house is worth).
http://www.worldpropertyjournal.com/real-estate-news/united-...
The part of the problem that nobody ever seems able to solve, is what you do for existing owners who are locked into a 30 year mortgage on their $1,000,000+ property. You would effectively be wiping out their life savings. There would need to be some way of appeasing the existing owner base, before you can ever solve the housing crisis.