Strength is just one factor for the bounty. There is also Threatlevel to WG and Civilians. But even with pure strength you can have a higher bounty than someone if you for example overpower someone with a higher bounty and win against that person. Your bounty will be higher than that person afterwards.
Luffy beat Doffy yet only got 100 Mio increase
Because he did an favor for the WG and this deed wasn't dangerous
He beat Kata and Cracker he again meddled with the balance and yes defeating those two makes him more dangerous due him interfering with the balance
Now for strength part yes it is for Marco and Katakuri both are way more peaceful than the BP yet got billion bounties
If you look at it danger act+strength=bounty
So how much is dangerous how much is strength?
For each character?
You have to look individually