I now that weight varies with the breed, but i'm curious as my nine month old male kitten is heavier than my one year old.
Just like people, it varies. Cats vary in size a lot. 9 lbs might be normal for one cat, 16 lbs. might be normal for another. the cat should have have a "waist" and not a lot of extra fat or a belly. Otherwise, he's just going to be a big cat.
Although the weight might vary, it is unhealthy for a cats weight to exceed 10 lbs. Overweight cats are prone to kidney and other urinary tract problems.
I would expect a 1 year old male cat to weigh about 8 pounds. If you are concerned that your cat is underweight, you may want to take it to the vet. There could be an underlying cause, such as worms and your vet will be able to advise you on this.
If they are different breeds that would explain it, also some like humans have faster and slower metabolisms.