You assume that developments that exceed the zoning constraints consist of dwellings that wouldn’t be produced elsewhere. Why? It seems to me that the demand for buying new homes is what determines the rate of new home construction. And if you can’t buy in one building or subdivision, you would look elsewhere, encouraging the development of other sites.
More clearly, how is it that you think allowing development outside the zoned limits reduces home prices? Are you saying developers will simply flood the market with so many homes that the price falls? That is not profit maximising behaviour for any one developer, or for all landowners/developers as a group.