K.,
I am not a mom of twins, but rather a twin myself. As is normal, my sister and I were born premature by about 4 weeks. Definatley not as small as your babies, but we did have some challenges growing up.
The thing I could contribute as far as "catching" up is this: I am 33 years old, 5 feet tall and 105 pounds! Do you think that is caught up? Truthfully, it isn't something that bothers me, it is all my sister and I have ever known. And I know God made me just the way He needed me.....and I am healthy. The problem with this seems to be with other people. I can not tell you how many times perfect strangers have commented on how tiny I am or how little I weigh. I have learned to say things like, well I am one of two so if I were full size I would be 5 feet tall and 210 pounds! It kinda throws people but I guess that is my intention. I think their comments on my weight and height are as rude as if I were to say to someone who is extremly tall and heavy, wow you are a giant and overweight!
I guess what I am saying is, your babies may never catch up with other children from a growth presepective, but as a Christian you'll have the ability to teach them that it really isnt important. The thing I know that God has taught me through this is that every single one of us is uniquily different and designed to fit the plan He had for us, part of that is the way we look and our size. Do your babies a favor and grab a hold of that now, so that when they are exposed to others, they will not be thrown by their comments. And dont be afraid to stand up for them in front of them, not in a rude way, but surely people know it is rude to comment on someone's weight or height.
May God Bless You and your family and keep them whole!