Etymologically, “giga” does make more sense as a generic ’big’ prefix than “mega”. I believe that the former basically means “great” while the latter means “mi

Etymologically, “giga” does make more sense as a generic ’big’ prefix than “mega”. I believe that the former basically means “great” while the latter means “mighty”, which is less flexible. “Tera” has the problem of being overloaded. It’s a homophone with “terra” which is a common prefix for things related to the Earth or ground. Also that lack of a strong second consonant hurts it. Etymologically it means “monstrous”, which is even less flexible.