Why would he blow past Hancock? Luffy and Law only got 500M for beating him
We saw with Zoro and Sanji you can have a bounty lower than the dude you defeated.
Because Zoro and Sanji are less dangerous than King and Queen who destroyed cities with fire and plagues.
If some ex Warlord got a 1 milliard + bounty (it's a spoiler for now so I can't tell his name) without doing a lot, Doflamingo has to have a 1 milliard + bounty with all the shits he did.
Brief summary of what Doflamingo did after je got his 340 million bounty :
- stole a lot of money intended to the Tenryubito
- blackmailed the WG to become Warlord
- attacked Dressrosa and became King of Dressrosa
- during 10 years, he got rid of Marines and WG agents thanks to Sugar's powers
- during those 10 years, he also became the ultimate broker in the Underworld
- destroyed Dressrosa during his fight against Luffy and tried to get rid of an admiral, vice admirals, a lot of soldiers and citizens with his Birdcage.
With all of this, I would not be surprised if he ends up with a bounty around Crocodile's the day Oda decided to update Doflamingo's bounty.