Seriously .. is it haraam or sinning to be fat/overweight/chubby ?

Allah Almighty tells us that He has provided us with abundance and to enjoy the life He has given us. He also teaches us moderation in all things. You might want to consider dieting, but it could be your genes. Being a little on the heavy side (or a lot) is not haram. Just eat many small portion. So long as the portions are small, you will not gain weight. If you start walking daily, you may lose some.