I read somewhere that lighter runners can perform better than others. Since I am not exactly thin (I am not fat either) I wonder if it has any influence on my speed or endurance. Is there an optimal weight for racing? If so, what is optimal racing weight for a 6 feet male?
There is no such thing as optimal racing weight. Your performance depends on your strength, endurance, stamina and your overall form. Your weight is pretty much irrelevant. Of course, if you weigh 300 pounds and most of it is fat that you cannot be as fast as someone with the same muscle mass who weighs 160 pounds. But this is an extreme case. The most important part of your body is muscle mass, especially in the legs. They propel you and if they are strong and well trained you will be as fast (or even faster) as the others.