Some hard data: bond enthalpies

$$\begin{array}{c|c|c|c} Bond & Enthalpy &Bond & Enthalpy  \\ \hline
C-C & 350 & Si-Si & 226\\
N-N & 163 & As-As & 146\\
O-O & 146 & S-S & 226\\
F-F & 155 & Cl-Cl & 240 \\
\end{array}$$

We observe fal in bond energy from carbon to nitrogen and from silicon to arsenicum. It is clear and probably have same reason: appearance of lone electron pair othe atom. There is a repulsion between lone pairs, that is in partially compensated by stronger bonds formed by smaller atoms (the atomic readii falls down to the end of the row). When moving from second to third row, this repulsion is reduced thanks to larger size of atoms of the elements of the third row.

It seems, that in case of As this effect is overshadowed by the fact, that lone pair is located on the S-orbital, isntead of hybridized $sp^3$ orbitals, and thus they repulse stronger. 

The stability of esters vs thioesters is ruled by different reasons. Sulfur is not as electronegative as oxygen, but when you consider an anion, sulfur anions have larger radius and are less prone to grabbing first positive charge they find. So sulfur anions are more stable intermediates (kinetically), and thus polar dissociation of thioester bond is easier.