Mihawk had a high bounty that Buggy before the freeze. Use your brain
Bounties don't increase addition-wise lol.
They're overall given.
For example, Luffy was 1.5b, Kidd wasn't even 500m, but both are 3b.
Having high bounty before doesn't really matter all that much into bounty given for your overall accomplishments.
It only matters if it's set raises like with SHP when they don't know anything about them so just slap them with 200m per head. Meanwhile Zoro, Jinbe and Sanji are given actual raises.