There have been cards where they've shown every fight on the PPV because of how quick they are. All depends on how long the fights take, especially the main event.
edit- And the main events didn't always use to be 5 rounds, so those cards wouldn't leave as much room for more fights if the main event went quick.
They didn't fill time with prelims between short fights because of the hour there in Brazil (like 2-3am). The show flowed a lot better with prelims rounding out the last hour instead.