If you create the foundation, do the plumbing, wire the circuits, cut all the boards, pound all the nails, set all the windows, mount all the doors, and lay all the shingles, you just get too tired to build a mansion. My own tiny cabin is a mere 1140 square feet, and I had some help. The people I have known who do everything for themselves are generally satisfied with something even smaller.
This general hypothesis is well-illustrated by the family of house wrens that has built an abode in my wood shed. As you can see from the picture below, the loving songbirds did not wholly plan for quadruplets.
