Pretty sure King is having a higher bounty than Marco for these reasons:
1. Bounty is not powerrelated
2. Marco is not "that" troublesome for the government as King is. Marco and Whitebeard werent raiding Islands, producing weapons...
3. King is in a way more violent crew. They will surely have way more dead bodies on there counting than marco, if marco has any overall.
4. King maybe has way more knowledge about the government than Marco, since he seems to be kinda related to Impel Down.
5. King seems to be from a pretty extinct species, which is probably extinct for a reason.