Here's why:

* Solubility Trends: Solubility of metal hydroxides generally increases down a group in the periodic table. This is because the size of the metal cation increases, leading to weaker attraction between the cation and the hydroxide anion.

* Barium's Position: Barium is located in Group 2 (alkaline earth metals) and is the largest of the common alkaline earth metals.

* Other Factors: While size is a major factor, other factors like the charge of the metal cation and the stability of the hydroxide anion also play a role.

Important Note: Even though barium hydroxide is the most soluble metal hydroxide, it's still considered a strong base and should be handled with caution.