It goes to an extent. But the developer is subdividing those lots before he ever builds anything. He's submitting his zoning plan to the county, parish, etc. with a detailed breakdown of what he plans to build.
He's building bigger houses because, as you stated, there's a family out there with the money to buy the big home.
The thing is that the family with the money to buy the big home could just as easily buy a smaller home and bank the difference. They don't. They take out the biggest mortgage that they qualify for and buy that home. I'll use a personal example for contrast. My dad's a doctor, had a very successful practice, so he was financially secure. He and my mom, a nurse, never bought a house that cost more than she could afford if she had to handle a regular nursing job. We had family friends in million dollar houses whose parents made less money than my parents. But my parents lived by the principle that they wouldn't buy any more house than they needed or that the lowest earner in the family could afford.
Not judging others but Americans don't follow that principle. They buy the most house they can. And then we wake up 30+ years later and wonder why housing is so expensive.
Tangentially, people are being sold a false narrative regarding equity, imo. Equity is meaningless if it's your family home. Yes, people can access the equity to pay for things but that access comes in the form of a loan. It comes with a cost. People would be better off buying the smaller home and investing the cost difference over 30 years. At least that way, when they needed the money, they wouldn't have to pay it back with interest.