I am not saying mihawk is not strong. Think about jimbei's case. Jimbeis' bounty explosion can just be explained because he is straw hat anymore. Same condition for mihawk . Mihawk is already more famaous than buggy but if the most known pirates create a something, of course marines put additional bounty for that. Also , we dont know previous mihawk bounty before becoming warlord. But we know Buggy. Logic.