Mine gained about 1/2 a pound every 2 weeks in that stage (yes, they were little oinkers)
I would have her reweighed at the vet's office, since that's the one that they will be using for all of her medication calculations, etc. 1.6 kgs = 3.5 lbs. Are you sure the vet didn't say she weighed 1.3 KGS, which would be 2.9 lbs? That would put the weight gain in the right scale, so to speak.