Actually, it's not. One of the guys in this chapter had a really high bounty because he wrecked a lot of shit AND was dangerous. They base your reward on how much damage you do, as well...
Luffy and crew don't really level up, they just slowly get stronger. For example, the only real level up Luffy gets is Gear 2. Gear 3 just makes a really big hand or foot.
Keep in mind that Luffy also didn't get stronger or anything in between the very beginning and the CP9 arc, he just gained experience. They only kept raising his bounty because he kept on proving he was worth it.