If it's just about the electrons being pulled closer to the nucleus, why doesn't the effect apply nearly as much to it's neighbors on the periodic table?
To extend on wiredfools remarks, the effect of non-full means lots of electron sharing going on. But if they're full, the (smaller) effects of relativity are the only effects so they're dominant. The neighbors are non-full so the sharing they do is way more important level than relativity level effects.
