Antimony and Berllium
The antimony and beryllium ions are so small because atomic number of antimony and beryllium is 51 and 4. Beryllium tends to lose 2 electrons to form beryllium ions (Be2+) so it forms 4 coordinated complex ions, it does not form simple ions. Antimony gains 3 electrons to become stable or form an ion (Sb+3) so these ions are small.
Both ions and atoms tend to grow larger as we go down the periodic table because protons are added and atomic number increases. So new shells are also added. The new energy shells provide shielding effect so atomic size increases going down the group in the periodic table.