Tom Anderson wrote:
I have been told that the cost of making new tube tunnels depends
on their size, with Crossrail-style NR-standard tunnels being more
expensive than classic LU-style tube tunnels. Is that true? If so,
why, and how big is the difference?

I can see why it might be greater - there's more mass to shift, more
surface to line - but not massively greater.

I think you may not have seen the actual figures. AFAIK these are
external tunnel dimensions (from Crossrail documents):
Victoria Line: 3.81m (older tube tunnels are slightly smaller)
Jubilee Line extension: 4.35m diameter
Crossrail: 6m diameter

Volume to excavate per metre of tunnel:
Victoria: 11.4m³ Jubilee: 14.9m³ Crossrail: 28.3m³
Surface area to line per metre of tunnel:
Victoria: 12.0m² Jubilee: 13.7m² Crossrail: 18.9m²

If it's only a bit more expensive, wouldn't it have made
sense to build the underground to be compatible with the
rest of the network, as, AIUI, in Tokyo?

London was the first city with deep-level tubes; the technology was in
its infancy, and a 12-foot diameter tunnel was probably as big as could
be managed at the time. Cost and the need for interconnections (e.g.
Victoria Line trains to Acton Works for major engineering work) were
factors which inhibited a subsequent change to mainline gauge.

--
Richard J.
(to e-mail me, swap uk and yon in address)