big, large, robust, name of tribe
壯 centers on strength and largeness: something big, robust, or imposing. The tribe-name sense is a specific historical application of that idea of a powerful group.
壯 is the traditional form of modern 壮. It combines 士 (scholar, warrior) with 爿, which likely contributes a sound element. The character expresses the idea of a strong, robust man or warrior.
A strong scholar-warrior 士 stands beside a sturdy wooden bedframe 爿, showing his big, robust build.
For ソウ, imagine a big, robust sumo wrestler named Sou: Sou -> ソウ, and his large frame fills the room.