Interesting concept! But surely more than 5 counties would be antipodal (and therefore furthest equal from) from some larger countries, though? (See this). Brazil, at its closest point, appears to be further away than Bolivia at its closest point, and Australia would qualify for United States's list - the US east coast is quite close to being antipodal to Australia's west coast.
Yes you're right but to simplify things for me, I only considered the geographical center of countries - I wouldn't know how to work this out considering the whole surface of countries!