Welcome on board.
I had a runt like that once, that still did not eat any solid food, not even soaked kibble, when 5 weeks of age. He started the same size as the others, but than got behind. When he finally started to eat solid food and meat, he slowly caught up. When about 10 weeks of age, he was hardly any smaller than the others and eventually did end up the same size. But that is only 1 indivual puppy. Usually I have them all about the same size. In your case, she still has to start eating solid food and there is no way of telling when she will do that. When she is 8-9 weeks of age, she really should eat solid food and you should see that she slowly catches up. If so, there is probably nothing wrong. But do make sure she is properly checked by a vet. Also when you have her at your home, have her checked by your vet for a second opinion. I worried sick about my little runt and it ended up being nothing to worry about. But that does not mean it is like that in all cases.