I’m not arguing against your position aerozol. (And I haven’t been changing Area: Opoutere to Area: NZ either. 
)
I can see 3 things that might lead an editor towards using “large area” rather than “small area”:
- " Area: Opoutere" would leave most users in the dark as to area. (Area: Opoutere, NZ would solve this.)
- Much less knowledge is needed to input “large area” , compared to the knowledge needed for “small area”.
- Those seeking to boost the importance of an artist might see “large area” national artists as likely to get more attention (achieve WP:notability?) compared to “small area” artists.