Digging a wider tunnel requires a bigger tbm, which adds little to the whole cost. Hs2 put out a paper, though I don’t think it compares sizes. https://assets.publishing.service.gov.uk/government/uploads/... The NYC example is terrible, because the new part of the tunnel is only short and the whole project costs a lot does not mean each mile of the tunnel costs that much.

I was first asking why they thought small tunnels are better, is that purely on financial grounds? Low cost, low benefit.