So technically, their radius should be the same. But it's not. It's smaller in this guy because it has a weaker shielding effect. In other words, the two electrons found on the outermost atom on the outermost shell experience more charge and so therefore are pulled closer. So, once again, the shielding effect is not as great in Beryllium as it is in lithium because the extra electron in Beryllium is added to the same energy level. The same two S orbital.