Here's the quote from Christopher Nolan, talking to Empire about The Dark Knight Rises:
"I think this is the biggest [movie] I've done... The biggest one anyone's done since the silent era, in technical terms ... It's not what you're used to seeing. I don't know when someone last did a film with 11,000 extras in a real environment. It is an escalation."
I remember, Chris! 11,000 extras in a real environment would have been but a fraction of Gandhi's extras; according to the Guinness Book of World Records the 1982 film used a record-smashing 300,000 extras to recreate the fallen leader's funeral.
Of course 'casts of thousands' were de rigeur in the days of the Biblical epic. 15,000 extras appeared in 1959's Ben Hur during the chariot race. But Gandhi still holds the record. I'm not sure if there were any movies after Gandhi that had a huge cast of extras (at least in one scene at one time), so The Dark Knight Rises can probably lay claim to being the biggest movie in 30 years. Which is impressive, but not quite as impressive as 'since the silent era.'