Healthy kitten weights

I have four and a half month old kitten that my boyfriend and I rescued from a shelter when he was (apparently) three months old. When we brought him home on August 10th, he was about 2.8 pounds and more underweight than we knew at the time. After some bouts of sickness and ringworm, he has grown to be a happy, healthy and bite-y little boy who loves playing fetch and scratching our couch. 

As of today, less than two months later, he is a whopping 7 pounds. We realize that he could be a little older than the shelter thought but this seems really big for a kitten his age. I've seen cat weight charts and he still has a waist, is very active, and doesn't look overweight in the least, so we're not worried that he's fat, but is this normal? Is it likely he's older than we think he is, or am I just destined to have a giant cat? 

It could be any one of the things you mentioned.  Usually kitten ages are pretty close because it's easy to get close to their age by their teeth.  He may simply be growing faster then average or he may end up being a big cat.

My family once adopted a supposedly 4 month old male kitten from a shelter, and when we took "him" to the vet we learned she was female and about 7-8 months old, but very undersized from malnutrition. She grew very fast over the next few months, catching up to where she should be. Mistakes happen. Of course your kitten could also just be big for his age. It doesn't necessarily mean he'll be huge as an adult.