I was surprised when I saw that Luffy’s newest bounty was less than Kaido’s, since in every previous arc Luffy ended up earning a bounty bigger than the villain’s (if they had one).
Then I realized the World Government may have seen the raid as a ‘team battle’ of sorts.
Regarding the bounties, Oda went the most logical route with them.
Kaido’s bounty is 4,611,100,000 Berries.
Big Mom’s bounty is 4,388,000,000 Berries.
If you add both of them you get 8,999,100,000 Berries.
Now, if you add Luffy, Kid and Law’s bounties, you get 9,000,000,000 Berries.
To be fair, the three of them fought Big Mom and Kaido so it’s fair to say that the three of them together were stronger than the 2 Emperors.
The bounty assignment logic is legit because we’ve already seen a similar situation in Dressrosa where Luffy and Law got the same bounty in Dressrosa (incidentally, this also means that the Marines considered Doflamingo to be a 1 billion berries pirate).