There is less muscle and more fat.
Women have earlier exhaustion and tend toward hyperventilation in an effort to compensate.
When standardized for level of fitness, there are no differences in rates of illness. There is some suggestion from aerospace literature and from dry chamber dives formedical hyperbaric therapy that women may be at increased risk for DCS during the first week of their cycle that is during the menstrual week. This phenomenon has not been studied in open water divers. It might however suggest that due to changes in hormones, electrolytes, vasomotor reactivity and peripheral vasoconstriction, and other unknown variables, women handle a gas load differently during the menstrual phase of their cycles. One might caution women who dive while menstruating to dive more conservatively jumping tables, doing shallower dives, and prolonging safety stops.