My dog naturally doesnt eat alot for his weight, but hes healthy active and happy, is this normal for some dogs?

That's good, I assume he's some sort of mastiff or dane, Definitely don't make him eat the extra two cups you DO NOT want any extra weight put on their joints. Be glad you don't have a glutton like my dog!! lol Mine would eat 5 cups in one sitting if I let him!

Dogs just have different metabolic rates, just like people. Some people eat one slice of cake and gain 5 pounds, some people can eat a whole cake and not gain a single pound. As long as he LOOKS healthy and ACTS healthy, Just do what works for him.

Just like humans, dogs can vary in their metabolic rates.

Right now I've got 3 dogs that all eat the same amount. One is 25lbs, one is 30lbs, and one is about 45lbs. All healthy and maintain their weight on the same amount of food.

Cut back the amount you offer to ensure he is eating the right amount for each meal.

My Shepherd is the same way. My vet said all is fine. All animals have a different metabolism. If the energy is there and you don't see any signs of pain or anything abnormal you should be fine.

Yup