i went to the vet last week with my 9 month old and she's 5.4 pounds. i asked him if she's big enough for her age and he said that every cat is different, some simply stay small and petite. my other cat was way bigger than her at that age. he is now twice as big as she is. i hope that now that she's spayed she'll fill in some more.
my two cats are also very different eaters. the big one eats what he get and as much as he can and has a tendency to obesity and the little one eats often but very little at one time. i think that has something to do with it...
The general rule of thumb for kittens is one pound per month of age, but it does begin to slow down at about 7-9 months. The ideal weight of a cat is determined by body frame and not age. My larger framed cats weigh 11-13 pounds and they're all at healthy weights. My smaller framed cats weigh 7-8 pounds and would be obese if they weighed as much as the others. You can use the body condition score card linked below to get an idea as to whether or not your cat is at a healthy weight.