Probably because people are only building McMansions - mostly because the way the market works - buyers are only buying new McMansions.
Between site prep, utility hookup, permits, and plans - there's probably close to a $30k fixed cost to build a 600 sqft house.
If the lot also costs $15k (usually $60k+) - you're looking at a $45k - $90k fixed cost.
Every house has at least one kitchen - you can build these for much cheaper - but the types of kitchens buyers of new houses expect cost at least $60k. Same for a bathroom - but you're looking at $20k.
Assuming the rest of the house costs $100 sqft - a new 500 sqft house is going to cost $50k + $60k + $20k + $45-90k = $175-225k. You want a ~20% margin - so this becomes $210k-$270k. At this point - why are you building a 500 sqft home? You bump it up to a 2bd + 2ba 1400 sqft home, and you run into the same problem. There's no buyers at the price you can build - so you build a McMansion instead.
Between site prep, utility hookup, permits, and plans - there's probably close to a $30k fixed cost to build a 600 sqft house.
If the lot also costs $15k (usually $60k+) - you're looking at a $45k - $90k fixed cost.
Every house has at least one kitchen - you can build these for much cheaper - but the types of kitchens buyers of new houses expect cost at least $60k. Same for a bathroom - but you're looking at $20k.
Assuming the rest of the house costs $100 sqft - a new 500 sqft house is going to cost $50k + $60k + $20k + $45-90k = $175-225k. You want a ~20% margin - so this becomes $210k-$270k. At this point - why are you building a 500 sqft home? You bump it up to a 2bd + 2ba 1400 sqft home, and you run into the same problem. There's no buyers at the price you can build - so you build a McMansion instead.