User blog comment:Kaido King of the Beasts/Chapter 836: Man of a Thousand Pounds/@comment-68.41.13.84-20160818141636/@comment-24442815-20160818145158

Bounties also equate to influence and power they hold as pirate as well as any collateral damage they do to civillians. Kid for example liked killing civillians while Luffy does not. The World Government has also covered up some of Luffy's accomplishments in the past thus making his overall bounty lower than it should be.