Weight has the greatest impact, due to their metabolism. The average speed of metabolism (the rate that the alcohol is being broken down) will be slower the smaller a person is.
People who weigh more will generally have more water present in their bodies. This water has a dilution effect on alcohol that is consumed, resulting in lower blood alcohol content. If two people consume the same amount of alcohol, the person who weighs more will generally have a lower blood alcohol content if other factors are equal.
