There is nothing inherent about women that would make them not work as hard as men. That's nonsense.
If you're talking about "putting forth as much effort", you're probably right but if you're talking about "able to perform as much physical labor in the same amount of time", you're definitely wrong.
Obviously there are individual women who are stronger than most men but on average men are stronger and can perform more physical labor.
It's not discrimination that lead to most loggers, miners and firefighters being male, it's biological differences.
There is some physical recovery for the woman after childbirth, especially depending on the method of delivery. However, the amount of leave goes beyond what's necessary for recovery.
They would probably just outright lose the employee if they didn't give what she deemed to be "Adequate" leave when a child is born. It's a compromise that employers make because it's better for them to wait a few additional weeks for the experienced employee to return than it is to hire and train a replacement.