They determined that athletes who do not use anabolic steroids had a maximum index of roughly 25. Steroid users might easily outperform that figure, typically by a large margin. This leads to the hypothesis that an FFMI of about 25 is a natural limit. However another study found no difference in muscle size between steroid using and non-using athletes. Nowadays there are many examples of extremely muscular people who did not use steroids. Some even claim they got bigger muscles by eating more than others! The answer seems to be that they reached their genetic potential for muscle size around age 20 and didn't grow anymore due to aging or other reasons. After this point, you can only get bigger if you use performance enhancing drugs. In conclusion, the maximum possible muscle mass for a male without using steroids is about 25kg (55lb). If you're very strong you could probably reach 30kg (66lb), but most people won't be able to go beyond this value. As for females, the maximum possible muscle mass is about 16 kg (35 lb). Usually males will have a higher percentage of body fat which makes them appear larger because of their heavier bones which weigh more too. Females usually have less body fat so their muscles look better. But regardless of the gender, we think it's safe to say that you shouldn't weigh more than 100 pounds (45 kg) without using drugs.