Small houses and gypsy wagons and small boats have something in common! Strength! Yes strength If one were to have a beam of say twenty feet long made from soft wood of say 3"x2" and it was supported at the ends it would bend under its own weight. This is fact. If the same piece of timber was cut to 1feet long supported also at the ends it would support a large weight. Look don't take my word for it go and do it for your selves.
This is why small is cheaper not only for the less materials smaller sizes are required as well. With this comes less maintenance because of surface area. Lets use our 3'x2' again.
Simple maths 3"x12"= 36" square inches 3 being the width and 12 being the length
Now multiply the length by 20 to to make it the original 20 feet long.
Simple maths again 3x 12x 20=720 square inches to maintain.
Or 720sq inches compared to 36sq inches.
This is only on one surface and there is six sides to a cube and hollow the cube and you have 12 surfaces to look after
36 becomes 432
720 becomes 8,640
There's the difference in area only and it goes across the board with everything including money.
Houses also have a cost not associated with the boat and the gypsy wagon.
Foundations and councils. This is another story.