I’m still surprised whenever a man picks me up with ease, especially if I know that they don’t work out regularly. It’s frustrating knowing that I could not do the same despite having focused on muscle strengthening for a year now. But muscle composition depends strongly on gender and there is a long list of how our muscles differ…
Men are stronger and have a larger cross-sectional area of muscle fiber. While it’s easier for men to build muscle, their muscles also deteriorate more quickly as they age. On the other hand, women are more resistant to muscle fatigue and have a higher proportion of oxidative muscle fibers and higher density of muscle capillaries. Men may be able to build muscle faster but women’s muscle dynamic strength builds more quickly (albeit from a lower base.) The reason may be partly because although womens’ bodies signal muscle growth like mens’, they also send out signals that hinder muscle growth after participation in resistance exercises. Another cause may be hormonal; with about 10 times more testosterone boosting muscle growth in men than in women. 
Why our bodies react differently to weight lifting is not as important as how we, as women, can harness the differences to our advantage. Most weight-lifting regimen are male-focused because men have had to build their muscle strength in order to dominate and survive throughout history.
The biggest change for women’s strength training routines should be to vary up the weights and repetitions. Many women only lift lighter weights and do many reps, often not even feeling a burning sensation in their muscles during the workout. This doesn’t fully exercise muscles and, therefore, significantly limits results. Women are scared of getting big and bulky, but if you do regular cardio workouts in addition to weightlifting then you will maintain a lean physique. To reiterate the studies above, womens’ muscle mass builds a lot more slowly and from a much lower base than mens’, so to be big and bulky you would have to do frequent isolation exercises regularly (which is what female body builders do.) Hitting the gym with your guy friends or find a weight-lifting routine just outside of your comfort zone is fine, just make sure to feel the burn!