Boxing is based on the box office draw and champions, so the champion will always get the biggest purse, because he holds the belt and is the main generator of the revenue.

In non-heavyweight bouts, or a fight where two challengers are fighting for the belt, then the purse is going to go to the biggest draw.

That's why all of that is negotiated before the fight.

If a nobody is fighting the champion, you think he deserves to get paid more than the champion if he wins?

Now in a rematch, the new champion or previous winner gets most of the purse or even split....It still depends on how big of a draw the fighter is, especially in boxing where gate revenue and pay per view matter. In boxing, there are no big nfl/nba tv deals or endorsements. Money is highly generated from the gates and PPV, so the biggest draw is going to get most of the dollars.