Hawkins was an Headliner as much as Garp is a Vice Admiral, you still can't get your head around that
Bounties weren't assigned based on who defeated who, otherwise Zoro would've got a bounty of 1,400,000,000 at least. Bounties of the crewmates are near-always proportioned to the captain's bounty. Zoro and Sanji didn't surpass King's and Queen's because Luffy didn't surpass Kaido's bounty.
Killer's captain has the same bounty as Luffy, he covers the role of Vice Captain and Combatant (basically the same role as Zoro), it's only natural that his bounty will be proportioned in a similar way. On top of that, Killer's bounty has always been 50% or 40% of Kidd's so 1,000,000,000 (33%) is the bare minimum and 1,111,000,000 (Zoro bounty) is the maximum cause he can't surpass that.