In my husband's mother's side of the family, the babies have been huge and the people are tall. My babies, though large, were not the largest babies in my husband's side of the family!!! My daughters weighed 11 pounds exactly and 12 pounds, 8 ounces. I love seeing babies when they are two or three months old so that I can guage how big my daughters were.
I did not have gestational diabetes for either pregnancy. I took the screen for the first pregnancy. However, because of my history with large babies, the OB ordered the full tests for my second pregnancy and confirmed that I did not have GD. Because of my interest in controlling the baby's size, I ate a modified GD diet for the first 37 weeks of my second pregnancy. (a few sonic limeades when nausea was bad, but mostly very low sugar)
My first baby was 11 pounds. I had severe hyperemesis to the point of vomiting until my throat bled. The hyperemesis lasted from 7 weeks in pregnancy and I was even vomiting during labor! I gained only 25 pounds in that pregnancy and lost the weight within a few short weeks without doing anything special. I did drink Sonic limeades and eat grits, so this would support the carbs theory; however, because of the hyperemesis I didn't eat much grits or limeade.
My second baby weighed 12 lbs, 8 oz. Again, I had hyperemesis (but not severe, just vomiting a few times a week with a lot of nausea) from 7 weeks on. I gained more weight (I'm guessing 35-40)but still have not lost that weight. This was the pregnancy where I ate the modified GD diet, having limeades occasionally when the vomiting got bad but staying low sugar and walking and swimmming as much as possible to try to control my baby's size.
I stopped following GD after 37 weeks when the u/s showed that the baby already weighed over 11 pounds... the specialist who did the u/s (not a regular OB, I forget what they are called) said she was probably significantly bigger, but their charts did not go high enough to make an accurate estimate with my baby's measurements.
Since my OB refused a natural birth and insisted on a cesarean if it appeared the baby was giong to be that big (and since I was looking for excuses to eat whatever I wanted) I gave up on the GD diet and ate whatever I wanted the last 3 weeks. So those last weeks of eating freely might have contributed to the baby's size. Plus, I didn't walk as much the last 3 weeks because of the foregoing excuse, plus, it was hot, I was humongous, and I just really didn't feel like walking.
So -- in conclusion: I think that genetics really do play a big role.