It has already been explained that the pirates bounty under the yonko is inflated by the yonko themselves.
And Buggy the clown besides being a yonko is famous for having been a member of the crew of the king of pirates. So it is obvious that the bounty of the L-hawk is for hiding in the shadow of that very famous clown.
That's not how that works.
Commanders get inflated bounties only when their captain does something truly relevant.
Hiding behind buggy's shadow doesn't get you a bounty.
I mean, take Daz Bones. I don't think he got a New bounty just because he has joined the Cross Guild.
Everything has already been explained in the chapter :
- Crocodile got his bounty because he has an Amazing logia and have intelligence and charisma to create dangerous criminal organization (and thanks to him the Cross Guild is funded by the Underworld)
- Buggy because he is considered as a mastermind to make crazy things like the Impel Down breakout and the creation (with Crocodile and Mihawk) of the Cross Guild
- Mihawk because he is just the WSS, with better sword skills than an actual Emperor (Shanks).
To be fair, oda is very inconsistent when it comes to WG hunting pirates and the reflection on their bounty.
Luffy defeated 2 shichibukais + invaded 3 federal properties + burn the WG flag + tagged along with Roger pirates' number 2 + punched celestial dragon.
How much all of those actions were worthy: 400 mil berries, keep in mind that gorosei already knew that "rubber" fruit was actually nika and still didn't do anything.
How tf did any of the commanders have bounties bigger than luffy after all that lol.
Because strength is also taken into consideration.
Whatever Luffy did, if a simple Vice amiral can take him, why give him a larger bounty.