Why are girls' physical strength and fitness limits lower than boys'?

For this explanation, I'm talking about the "average" boy and the "average" girl. There are a number of reasons, but the major two would be size and hormonal levels. Boys are on taller and heavier than females. This means that they'll have more muscle in total than females, and will be able to lift and move more with that mass. Their cardiovascular system is also adapted to deal with this extra mass, meaning it can output a greater volume and result in an increased endurance. The reasons for this would stem from lower levels of testosterone in females. Testosterone encourages muscle growth and speeds recovery after an intensive workout meaning that a typical male would gain a greater amount of muscle for the same amount of training as compared to a female. The result is that the average female has less muscle mass and lower endurance. Again, we were talking about averages. A top female marathon runner will have greater endurance than 99.99% of men, and a serious female weight lifter will also be stronger than the vast majority of men. Don't let averages lead to stereotypes that women are "weak" or anything of that manner.