thats genetics.. as Kratos said you can do nothing about it. I have thin bones too but i do not pay so much attention to this now as i did. Also some say people with thin bones look better overally when they have enough muscle mass, compared to others with thick bones (dont blame me for this one, some people do say it). But you are right on that one - people with thicker bones are able to lift havier, but i do not care about that as long as i look good, you shouldnt care about it too unless you are a powerlifter.