The 'most popular' and the 'most listened to' DJs aren't necessarily the same. DJs who broadcast in the 'breakfast slot' usually get higher audience figures than those who broadcast later in the day, simply because that's when most people are listening to the radio. So Chris Moyles, Chris Evans, Shaun Keaveny, Christian O'Connell, Johnny Vaughan and Lisa Snowdon are almost guaranteed the best listening figures, even though a poll of listeners to find the most popular DJs might find, for example, that Nick Grimshaw, Steve Wright, Steve Lamacq and others are actually better liked by listeners.
Most DJs are self-employed. They (or their agents) negotiate individual terms with broadcasters. It's likely that broadcasters will recognise that they have to pay more for the top breakfast DJs (both as a consequence of the higher listening figures and as compensation for the DJ having to get up at around 3am on every weekday morning!).
Chris