Ok, you can’t make a newborn body fit 6-9 month clothes even if you weigh them heavier. You would need to use a larger body which might not look right with those limbs. I have made Kitten and Cozy into bigger babies. Basically I find that the newborn kits and bodies only fit some newborn clothes and some preemie ones so they aren’t the size of the newborns in most of my family. I wish there were more kits in a true three to six months size.
Back to your problem, which kits are your babies made from? Did you put weight in the arms, legs and head? That makes a big difference in how they feel in your arms. You didn’t mention it so I am wondering if she told you that.