Sometimes it's simply genetics and there's not much else you can do, but try to be as preventative as possible
Brushing daily, etc. There are some products out there which can aid. HealthyMouth is one that's a bit pricey, but approved by VOHC, and has had test trials.
I will say people who feed Prey Model Raw often rave about the dental benefits.
I'm not sure a pre-made raw or homecooked would be that much more beneficial... though I do know that a lot of people who feed premade say their dogs look better, because they have less carbs and preservatives, etc.
I agree that a larger kibble will not help, if anything it could make it worse. I definitely don't believe kibble helps keep teeth clean at all. It's the equivalent of a person expecting hard pretzels to keep their teeth clean every day.
Good luck
