My little girl was 0.4kg at 13 weeks (0.88lbs) when she was weighed at the vet but she was a super small runt. I brought her home at 14 weeks so she wont have been much heavier than that. Thankfully my girl was in proportion very affectionate and outgoing and a good eater just sadly she was on the small side.
She's not huge now 22 weeks 2.2kg (4.85lbs) but she's grown a ton since I brought her home. I had her on dry down all day (80% meat protein no grain no filler) and she was also on 4 small meals per day of high protein wet food (with a small kitten feed as high a quality as she will eat and don't limit amounts on wet food either feed her whenever she is hungry you want her to grow) with added Nutrical kitten high calorie vitamin paste. I would check though that your little girl is actually that age but also keep an eye on her to make sure she doesn't have any health issues as to why she's that size, it's true that sometimes in a litter they can get one super small kitten especially if it's a large litter (my girl was one of 5 and she's also from show breeder where all her siblings where normal sized) did you see any of the other kittens? Where they all small?
http://tomlyn.com/products/dog/vitamins ... -nutri-cal®-dogs
As you can tell she's grown a lot in the time since I brought her home but it's been a lot of hard work. I've since moved her over to raw feeding as my boy has a chicken allergy and can't seem to stomach dry well so it was easier to switch both over to raw although she's a little piggy and loves her meat so works out better for her anyway.
Arya 14 weeks
when I brought her home with her tiny kitty bunny and my hand for size
Arya today 22 weeks
She's still a bit smaller than average but nothing to worry about really and I still expect her to be around 8lbs full grown with how she's growing and filling out now
hand again for size now