Bookies major basis is making a betting odds are how the fans see the game. Here, despite Stephen Fulton is the champion, still Inoue is a more popular because he already cemented his name when he become an undisputed bantamweight champion, and not only that win but most importantly the manner on how he won those fights.
Well, bookies look at the following factor:
1. Past performances of the fighters, including their record and recent results
2. Current form of the fighters, including their training and preparation for the fight
3. Injuries or other factors that may affect the outcome of the fight
4. Public perception or hype surrounding the fighters
since Inoue had better recent performances and records, bookies will really consider him as the favorite. Also, with the public perception that Inoue despite climbing weight, will beat Fulton.