If that was the truth then Sanji wouldve gotten the higher bounty in Arlong Park.
Look at the bounty raises for Ennies Lobby....Sanji got 77 mill increase for beating Jabra and Zoro got a 60 mill increase for Kaku. And dont say it was because....it was Sanji's first time, because Luffy beat Lucci and got 200 mill increase.
Jabra was deemed as a more dangerous foe. And even if he was weaker ( which he wasnt)....the WG doesnt know this, since the new dourki systems were still unknown only to the CP9. They still thought Jabra was stronger and more threatening than Kaku. Which is correct anyways....
So again it has nothing to do with opponents and getting specific bounties because of it.