Body size is the first factor. Women are, on the average, built smaller than men. Therefore, equal amounts of alcohol cause a higher blood alcohol concentration in women due to the smaller blood volume. Body composition is the second factor. The average female carries more body fat than a male.
Body fat contains little water.
When consumed, alcohol dilutes in water. Therefore, a female has less body water to dilute alcohol in, causing a higher blood alcohol concentration, even if two equally sized men and women drink the same amount of alcohol. The third factor is a metabolizing enzyme that helps rid the body of alcohol called alcohol dehydrogenase. Women have less of this enzyme than men do. Therefore, more of the alcohol that women drink enters the blood stream as pure alcohol, and stays there.