From what I gather, the credits made by bounties and total number of kills isn't indicative in relation to ranking. Just that you have to turn in the bounties to add "credit" towards rank progression.
You can get a Novice with a bouty of 10k credits and an Master with a 2500 bounty, but if you're an Expert, the Novice kill doesn't count towards your rank progression, yet the Master one does. So you can say that your income from bounties does matter, and even the kill count, it's just that not all of it does.
Basically saying, if you want to increase your combat ranking, go after bounties equal to or higher than your current rank. An supposedly the further above your current rank the target is, the faster you will progress.