how much weight a pregnant dog with only 1 puppy will gain?

i have a gsm shepherd female whos got mated 48 days ago , her weight isnt increasing much only 1 or 2 kg increase, she used to weight 26-27kg and now she weights 28-29 , her breasts arent that much swelled they are a very little larger than before but still not that big i had a sonar done for her 3 weeks ago , the doctor wasnt very sure about her pregnancy, and there isnt any other doctor in my area to do a sonar for dogs. so my question is , if a dog pregnant with only 1 puppy will he become big later in pregnancy same as having more puppies? and about her weight how much weight a dog will gain if only 1 puppy in there. thanks