There are a whole host of reasons on a global scale, a few of the key ones include:   A generally lower level of physical activity, this has been observed worldwide, yes some conservative countries actively prevent women from outdoors activities, but even in "progressive" Western countries this is seen. For example in the UK, according to Sport England "There is a significant gender gap, with two million more men than women exercising or playing sport regularly."   A lot of women tend to manage weight badly during pregnancy, eating more than they need to, cutting back on physical activity, and then not recovering from this afterwards. I'm not a women so I can't speak from experience, but pregnancy does this to a lot of women, especially because there is a popular perception it is normal or even inevitable for kids to make you fat.   Equality in portions. A lot of foods and drinks are served in standard portion sizes, and the average woman has lower caloric requirements than a man. This is only anecdotal, but I certainly see a lot of women who end up eating almost the same portions as a man who is 6" taller.