when they play best of 5 sets this will become true
not necessarily as their physicality is inherently less, look at that picture above or Sharapova.
Even women players that resemble men (Navratalova, Serena, Amelia Mauresmo) have nowhere near the strength of endurance of a male player.
so when you equate these natural differences then the levels of effort is comparable.
as I say go and watch a female match between close seeds on an outside court and see how much effort they are putting in.
The main problem is lack of strength of depth in the womens game, leading to a lot of short matches between mismatched players.