Kaido had a higher bounty because he had a massive army + commanders + territory.
Mihawk holds that bounty because of how powerful he is + his title of WSS.
If you want to argue about the "cross guild" then how come a so-called commander of Buggy (presumed Yonko) gets that insane bounty while King, Kata, and other commanders have bounties that don't exceed 1.5 billion?