I didn't expect the answer to last long, well done
@Calthrop
Argentina & Buenos Aires is the correct answer. The answers between Russia & Argentina are:
Canada - Toronto/Ottawa
China - Shanghai/Beijing,
United States - New York City/Washington DC
Brazil - Sāo Paolo/Brasilia
Australia - Sydney/Canberra
India - Mumbai/New Delhi
FYI, the country ranking is in land & water mass although if one were to count only land mass, Canada would drop a couple of places but the answer to the question would have remained the same.