It’s a combination of factors. Power is only one part.
If i am willing to give anything i will say maybe 5% is others and 95% is power.
And the reasoning is simple. What is a bounty? A bounty is what the wg is willing to pay a bounty hunter to kill that pirate.
So picture this, usopp (east blue usopp) gets a pistol and goes on a killing spree, he kills a whole island worth of people with that pistol. So because you love people and are a good guy you put a 100m bounty on his head. Yosuka and johnny, or even zoro, goes, kills usopp, has the easiest fight of their life taking out fodder, now you have to pay 100m for 2 second of their life.
And we already have tons of examples proving i am right.
Look at what buggy did. He essientally did exactly what i had usopp do. He used his very minimal power to take out a whole island of fodder citizens (mostly). Yet his bounty was 16m which is reasonable to get someone of east blue zoro's strength to take him out.
Then we have luffy. If bounty was based on threat in any way, luffy's bounty in east blue would be zero. All luffy did in the east blue was beat up pirates and one corrupt marine, hell the marines should of offered him a comission not a bounty, yet he had the highest bounty to come out of the east blue at the time.
Also, there is only so much a bounty ahould reasonably go up for the same pattern of behavior.
Remember my last example of usopp, he kills a bunch of civilians, ao you increase his bounty to add a little extra incentive for bounty hunters to go after him. Then he kills more civilians, are you going to add another incentive? Why would you? You already knew he is a civilian killer. Like if bm blows up 2-3 more islands, we already know she blows up islands, if we didnt like it the original incentive should of been alot higher.
Also, since it goes without saying power is a large part of a bounty. A bigger incentive (for example adding 5m because the pirate kills civilians) could actually do the opposite and discourage bounty hunters from going after the pirate. We have even see how pirates use bounties as badges of pride.
So think about this. Look at buggy. His bounty was a low 16 mill...ish... imagine adding an extra 5m to his bounty. Now he has the same 21m arlong has. Buggy went from being an average pirate to being the strongest pirate in east blue, without gaining any actual strength. Bow do you think that will attract or discourage bounty hunters from going after him?
Then obviously we have the tired examples. Like robin, girl with the power to destroy the world, just saw the wg kill every one she ever knew (so the motive to destroy the world also) only worth 79m not even worth as much as luffy was for punching really hard. And really to drive this point home, at that point all luffy had really done was take down morgan (which the marines thanked him for) and escape smoker he was really just a bounty hunter at that point he hadn't killed civilians or challenged the marines. Although drum island would beg to differ..... either way a guy who punches hard shouldnt be worth the same as a girl with the means and motive to destroy the world, yet he was worth more.
Also, look at how ID was divided by bounty level. Could you imagine how halirous it would be to see a guy with the power of a 10m bounty, yet imprisoned in level 5 for all the civilians you killed....
BB "fight to the death and i will allow the strongest to join me"
Prisoner "wait, i dont want to do that, i am actually not that strong i am just here for poisoning a water hole"
Prisoner 2 "ya and i am here for.... well i was a film executive working for the weinstine company"
Prisoner 3 "and all i did was run drugs, but like it was alot of drugs"
Prisoner 4 "i am only here for tax evasion"
BB "wait is anyone here actually strong?"
Prisoner 5 "i did things to kids"
BB "ok who ever kills him can be on my crew"