It's not just genetics. According to the department of pediatrics at the Stanford University School of Medicine, it's also the signals they give when they watch their children eat.
Pediatricians reported in the Journal of Developmental and Behavioral Pediatrics that children who eat too much and too quickly are responding to signals from their mothers. When they videotaped 77 children, there was a clear association between how fast and how much the child ate, and the comments and signs they were getting from their mothers.
After all, remarks like "finish everything on your plate," and actions like serving seconds without being asked don't always instill the healthiest choices.