[James]

As other people have said it's because Big Brother is on for longer so the audience is spread out more over the series. More people watched Big Brother over its whole series than will watch I'm a Celeb - or X Factor for that matter. That's why Channel 4 won't cancel BB even though the ratings were down a bit this year.

And the other reason for higher individual ratings is that ITV carries over a larger audience from its other shows like Corrie.