Heavier people seem to be more prone to cognitive problems like Alzheimer's disease, and now researchers know why. It seems that packing on the pounds reduces the size of your brain, according to a recent study. {relatedarticles}Clinically obese people - those whose body mass index was more than 30 - had 8 percent less brain tissue than those whose BMIs fell in the normal range. Overweight people who participated in the study - those whose BMIs were 25-29.9 - had 4 percent less brain tissue. The loss was most significant in the areas of the brain that control decision-making and memory. Brains in overweight and obese people aged more quickly as well.