Hi Russian Blue,
welcome here! You must have loved to see Egypt today -she could be a Russian Blue, don't you think?

Regarding Nakita:
ok she is a Russian Blue and no Maine Coon. Russian Blues are maybe a little bigger than Abys (I have an Abyssinian and a mix) but with every cat there are larger and smaller purrsons as well. Filou, my Aby, is 8 pounds since he is 15 months old. But he's male. His cousin Xeline, a girl, is only 6,5 lb (3 years old). If you look at that 3 pounds for 6 months is small but not impossible. At that age most cats are mini because they move so much.

If I read what they write on the cat food boxes, I see that most cat food companies recommend about double the amount my boys eat.

Filou, the smaller, eats more than Tigris the bigger (13 lb). Both keep their weight now for more than 5 years although I serve small amounts of food whenever they ask for it.
I know that there are pets (most dogs but even cats) who do not stop when they eat and who will get really fat if you are not aware of it most of the time. You may be as happy as I am because Nakita only eats as much as she wants.
You could add some treats like malt paste etc.

If you feel you can trust your vet, then she's just a skinny teenager. And if she is playful and happy then I guess she's ok.