They’re not exactly forcing builders to build small, but making it more challenging to build large. Since 2002, any new home or addition built in unincorporated Marin County, California, larger than 3,500 square feet must be designed to use no more energy than a 3,500-square-foot home.
Maybe 3,500 sq. ft. is not your definition of small. But relative to many new homes, it is.
The law essentially forces builders to build with more energy efficiency as a goal, through the use of materials and alternative types of energy.