Richard J.:
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...

I don't think there was any technical problem with a larger tunnel;
it just wasn't considered commercially necessary. Remember, the Great
Northern & City opened in 1904, just 14 years after the City & South
London, with 16' tubes intended for main line trains (which, in the
end, did not arrive until 1976).
--
Mark Brader "People with whole brains, however, dispute
Toronto this claim, and are generally more articulate
in expressing their views." -- Gary Larson

My text in this article is in the public domain.