This is something that has really been bothering me. Why the hell does Demalo Black have a higher bounty than Arlong?! It's simply a travesty that Arlong has a lower bounty by 6,000,000 than this loser. This has made me think of a few theories as to why this happened.
- Bounty overinflation: Since bounties don't measure strength rather threat level and Arlong is based in the Four Blues while Demalo is in the Grand Line, maybe the Marine's are over inflating the bounties in the Grand Line simply because the region as a whole is full of pirates and they are willing to shell out their pockets to clean it up there. Arlong was threatening to take over East Blue, but maybe the marines don't see that as very threatening.
- Bounty Evolution: Bounties have simply gotten larger as the series has gone on. At the beginning 20,000,000 was a large amount, but now it is next to nothing. That's also the reason I think Crocodile had such a low bounty compared to the rest of the Shichibukai, because he was introduced in the time period of the series where 80,000,000 was simply a ghastly amount.