thats correct, but id guess the first check would be wrong then! i wouldnt worry so long as she doesnt look overweight.
in a chi i belive its IMPOSSIBLE (even in an adult) to increase the weight that quickly, thats more or a great danes increase weight.
1kg is the equivalent to 2.2lb, so at her first visit she would have been 2 1/2 lbs there abouts and then 1 week later almost 6 1/2 lbs...i VERY much doubt it! its very likely that the first vet was wrong or this last weight check was wrong, at 6 1/2 lbs at 12 weeks shes gonna be about 8-10lbs full grown! (3.5-5kg)
thats a big chi, but somewhere the weight has to be off, unless bellas mixed, 4lbs in 2 weeks is more like a cocker spaniel or even biggers groth rate.
if she had put on that kind of weight you WOULD have noticed...
if the vets fine with her weight i wouldnt worry, a vet WILL tell you if your dogs overweight.
its more likely that she was 2.1 kg at the first weigh in...
and welcome aboard Ruby