Question

Why the decrease in size between Li and Be is much greater than that between Na and Mg or K and Ca?

Answer 1

In sodium and magnesium, the space closer to the nucleus is already very negatively charged, repelling the outershell electrons from getting much closer. But in berylium there aren't any electrons to interfere with orbital size decreasing. So, large difference is observed in $Li$ and $Be$ compared to $Na$ and $Mg$.