Weight (体重) is one of the taboo subjects. In America, it is OK, or even good, to be thin but it is an embarrassment (尴尬) to be overweight. In fact, the thinner you are, the more beautiful you are thought to be. You’ll find that most models (模特) are nothing more than skin and bones. A woman who is thin is easy to become popular. Of course, there are lots of people do their best to be thin. They may use all kinds of ways, such as doing exercise, or dieting (节食). But, sometimes, dieting may bring about some healthy problems.
Americans are very careful about their weight and most of the time they do not tell others how much they weigh... even if they are quite thin. Therefore, you’d better not ask about it. But if you really want to talk about this subject, it’s always safer to say, "Oh, it looks like you have lost weight. " However, if you would like to be honest and polite, I suggest that you should choose your words carefully, perhaps by saying, "Hey, you look great. Very healthy looking. " And always remember, you can not use the word "fat".
Those thin women are more popular than those fat women.