This is the way I see it. Many may disagree.
It might cost $50,000 more to build a 30sq home as opposed to a 20-25sq home.
So if you build the 20sq home, but want to upgrade to your 30sq home in a few years, how much would it cost in selling fee's/stamp duty to upgrade. $30,000 or more I would imagine in most cases. I'd rather have the home I want upfront(as long as repayments are not an issue), than get the secondary home, and "waste" $30,000 or more on upgrading in a few years.
That said, most people could be happy with the 20sq home for life. But my point relates to those who actually want the 30sq home in a few years.