metabolic rate from the mother's eating before birth

I’m wondering why some people struggle with weight gain of 30+ lbs and some only gain 10-20 lbs. I’m in the 20# group and my mom was a big eater who never went on a diet and is thin. I wonder if that somewhat weight proofs a person. My sister never dieted like I did and stayed slim while being able to eat pizza or cake when she wants. I on the other hand bought into the gym and diet mentality and here I am paying the prize. I used to read about indigenous people who were lean and became fat on a western diet. I wonder if they were in a calorie restricted state in their original environment and had low metabolic rates to begin with. And the blame on rich western food is misinformation??
I found this great article by a clinical psychologist who specializes in eating disorders. He is a great resource to back up Matt’s claims. I don’t really need back up though, since my experiences show that he is right on.
http://feast-ed.org/Portals/0/Documents/Articles%20for%20FEAST/Dangers%20of%20Dieting%20a%20Body%20Adapted%20to%20Famine%20-%20Shan%20Guisinger.pdf