Why do beryllium compounds often have covalent bonds instead of ionic bonds?

Most beryllium compounds are molecular and exhibit covalent bonding rather than the ionic bonding typically associated with other alkaline-earth metals.

This behavior is primarily a consequence of the high charge-to-size ratio of the ${\text{Be}}^{2+}$ ion. Because the beryllium ion is very small and carries a significant positive charge, it exerts a strong polarizing effect on the electron clouds of neighboring atoms or anions. This intense polarization distorts the electron distribution between the atoms, shifting the bond character away from purely ionic toward covalent.1

While the chemistry of heavier alkaline-earth metals is generally well-interpreted through an ionic model, this model is less satisfactory for beryllium and magnesium. The covalent nature of many beryllium complexes is evidenced by their ability to be extracted from aqueous solutions into organic solvents, a property characteristic of molecular substances rather than ionic ones.1