Hi and welcome!
Not sure if I can be of help as such but when I got my two kittens from the "breeder" they were very underweight. Rio weighed just over 500g and Chilli was 700g. They were 9 weeks old. I suspect that they weren't getting enough milk from mum and possibly they were early born as their mum had a prolapsed vagina and an emergency c-section. Anyway, it turned out they also had Tritrichomonas Foetus (TF) which took around 7 weeks to find out - mainly because you are back and forth to the vets trying different things before finally getting a positive result for the TF.
They had the two weeks of Ronidazole capsules and have been fine. They are now putting on weight steadily and have a huge appetite.
In your case, it may just be that they are a little behind and will catch up - or it could be that they will just be small. The females are usually smaller than the males.
Hope this helps
As long as she is healthy and happy, us girls always say "size doesn't matter"
Lisa
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