My twins are 6 weeks old now. They were 5lbs. 13 oz and 5lbs 14 oz when they were born. I'm 5'6" and DH is 6' and I was able to have a vaginal delivery. I only gained 23 pounds total and looked all belly. I don't really know secrets for higher birth weight.....mostly to take it easy as to prevent pre-term labor. That's probably the best advice. And I had a duola praise me for eating peanut butter saying that's good for protein and weight (I never heard not to eat it to prevent allergies to peanuts/peanut butter in the kids as another thread mentions).
My sister also had twins....8 pounds each

. She's only 5'5" and gained 30-35 pounds. I don't think she did anything special other than holding out until her due date.
Congrats and Good luck!!