Quote Originally Posted by Carnage
I dont know I feel it strange for Luffy to get such a large boost in bounty without becoming substantially stronger. He hasn't mastered any form of Haki yet nor introduced any new attacks forms really. Not that I doubt his head will raise several hundred beli after breaking out of Impel Down and assaulting HQ, but just saying.
Oda has already explained that Bounties aren't proportional to the pirate's strength but to how much trouble they cause for the world government

Now since his last one Luffy has defeated a shichibukai, punched a tenryuubito, fought an admiral and escaped, penetrated and escape from Impel Down along with hunderds of other inmates including some with bounties over 100 Mil, is now fighting along with the most wanted pirate in the world and had his connection with both Dragon and Ace exposed to the entire entire world

That's one hell of a shit list to ignore