Hendrix is 2.5 pounds and I swear to you he didnt gain an ounce after 6 months old.... he just stopped growing haha. He is super healthy, but he just stopped growing.
Lucy is 7 months now and she just went to the vet and weighed 4.6 pounds and the vet said she might fill out a little and gain a couple of ounces, but she isn't actually going to get any bigger.
Every dog is different, but in my case they can definitely stop growing younger.
PS 6.4 pounds is PERFECT!!! I wish mine were bigger, they are so tiny that I get scared they are going to break hehe.