The venue itself is larger in square footage, roughly 550,000 compared to Moscone's 300,000. It also has a larger capacity for people and seating. That said, actual attendee count is likely to remain at around 5,000 people (per Gruber and iMore). The extra space does allow for more Apple engineers to attend and be present for labs, etc.

Also keep in mind that last year they held the keynote in the Bill Graham Auditorium in San Francisco in order to support the extra amount of press, etc. that attends just the keynote. The San Jose center will be able to accommodate that extra amount of people who then disappear after the first 3 hours.

So conclusion, yes, bigger venue, but they aren't using it to increase attendee count.