Ehh, Blackbeard was an unknown pirate with a 0 bounty. He's still growing, and chances are his 2.x Billion bounty is going to skyrocket once he gets whatever he's trying to get before the Marines do. Mihawk just has a former bounty that will be reinstated. That could be 3-4 Billion. If Luffy is defeating a 4.6 Billion pirate this arc, nothing is that hard to see at this point