I really didn't "get" the story behind the build until just before I left Seattle Center and I talked to Jason, one of the Americorps workers who was making bricks.
Making bricks for the World Home
The small "World House" (about 12'x15') is typical of houses Habitat builds in third world countries. Currently there are many of these structures in Cambodia and other Southeast Asian countries. This structure would house about 6 people. Even though the climate allows for much more outdoor living, it is very small by American standards. The workers were making bricks for the structure's facade.
The larger structure "Home for the Immediate Future" was also relatively small compared to a McMansion. Jason said it is designed to have moveable walls. Except for the "wet areas"--the bathroom, kitchen and laundry area--the walls can be repositioned to allow for expansions and contractions as the family grows, then contracts.