1- Weight training and workout makes you bulky and masculine. Women cant produce testosterone as much as men , this is a fact so it is impossible for women to gain huge amounts of muscle mass naturally. Unfortunately, the image that may come to your mind is that of professional female bodybuilders. Most of these women, unfortunately, use anabolic steroids (synthetic testosterone) along with other drugs in order to achieve that high degree of muscularity. In addition, most also have good genetics coupled with an unbelievable work ethic that enable them to gain muscle quickly when they spend hours in the gym lifting very heavy weights. Believe me when I say that they do not look like that by accident. Women who conduct weight training without the use of steroids get the firm and fit cellulite-free looking body that you see in most fitness/figure shows these days. 2- Exercise increases your breast size. Sorry girls. Women’s breasts are composed mostly of fatty tissue. Therefore, it is impossible to increase breast size through weight training. As a matter of fact, if you go below 12 percent body fat, your breast size will decrease. Weight training does increase the size of the back, so this misconception probably comes from confusing an increase in back size with an increase in cup size. The only way to increase your breast size is by gaining fat or getting breast implants. 3- If you stop weight training your muscles turn into fat. This is like saying that gold can turn into brass. Muscle and fat are two totally different types of tissue. What happens many times is that when people decide to go off their weight training programs they start losing muscle due to inactivity (use it or lose it) and they also usually drop the diet as well. Therefore bad eating habits combined with the fact that their metabolism is lower due to inactivity, and lower degrees of muscle mass, give the impression that the subject’s muscle is being turned into fat while in reality what is happening is that muscle is being lost and fat is being accumulated. 4- Weight training turns fat into muscle. More alchemy. This is the equivalent of saying that you can turn any metal into gold; don't we wish! The way a body transformation occurs is by gaining muscle through weight training and losing fat through aerobics and diet simultaneously. Again, muscle and fat are very different types of tissue. We cannot turn one into the other. 5- Women only need to do cardio and if they decide to lift weights, they should be very light. First of all, if you only did cardio then muscle and fat would be burned for fuel. One needs to do weights in order to get the muscle building machine going and thus prevent any loss of muscle tissue. Women that only concentrate on cardio will have a very hard time achieving the look that they want. As far as the lifting of very light weights, this is just more nonsense. Muscle responds to resistance and if the resistance is too light, then there will be no reason for the body to change. Source