Why does Forest City need to be so big?

Because scale is what pays for infrastructure. This is the single most misunderstood thing about property development in modern Britain.

A 2,000-home development can't fund a train station. A 10,000-home development can't fund a hospital or a reservoir. That's why Haverhill, after decades of piecemeal growth, has none of these things—every scheme was individually too small to justify them, so the developers built houses and left.

A city of a million people flips the equation. At that scale, a railway, a reservoir, hospitals, a tram network, and a 12,000 acre nature reserve all become affordable—funded by the value the city creates rather than by taxpayers. Britain used to understand this: it's how the Victorians built, and it's why we're proposing one properly planned city as an alternative to 340,000 homes smeared across the East of England as village extensions that fund nothing.

Building small is how you get all of the housing with none of the benefits.