Who knows what went on.
Folk were going on about how Overeem was paid more than Miocic - and someone pointed out he has a much more decorated career in MMA and Kickboxing, so the cheque book is going to have more digits on it in comparison to Miocic's. Ngannou getting 500K was a bit of a surprise, but we'll never know how these talks go when it comes to money.
I remember when Dana was going ape shit because Holm didn't want to sit out and wait for Rousey to come back for the rematch and talked about her pissing away millions while bashing her manager. A fight with Rousey was a life changer money wise, but at the same time, Holm would have being sitting out for months - and Rousey could have pissed off and not fought again.
When it comes to negotiating more money, the fighters are probably better off doing it themselves.