Tom Anderson wrote:

For northbound trais to be be full at Victoria, Brixton, Stockwell,
Vauxhall and Pimlico would have to be generating as many passengers
as all
the Northern line stations from Morden to Kennington put together, or
the
Piccadilly line stations from Cockfosters to Caledonian Road. I don't
know
those areas terribly well, and i realise that at least some of them
are
very densely populated areas, but that seems quite surprising. Is it
because there are few people getting off the line before central
London? I
guess a lot of Picc passengers switch to the Vic at Finsbury Park,
and
perhaps Northern passengers to the Vic at Stockwell! Or is the
Victoria
line picking up a lot of passengers from the suburban railway
network?

Since Brixton's NR service is pretty feeble, IME far more people
(myself included) join the Victoria Line at Brixton from buses. It acts
as a (pretty unpleasant) parkway station for a huge area of tubeless
London, and I think is the busiest station outside Zone 1. If only it
had a decent bus interchange (like Hammersmith)!

As Annabel and others have already pointed out, it also takes a large
amount of the Northern Line's central London passengers at Stockwell!