> Therefore, In WDI 2016, there is no longer a distinction between developing countries (defined in previous editions as low- and middle-income countries) and developed countries (previously high-income countries). Regional groupings (such as “East Asia”) are now based on geographical coverage rather than a sub-set of countries that were previously referred to as developing. In some occasional cases, where data availability or context have dictated it, we’ve excluded high income from some charts or tables, and we’ve indicated that in the footnotes.
That's when it defined China as "upper middle income." That's based on Gross National Income per capita, as the World Bank calculated it. The thresholds are updated every year and adjusted for inflation.
By the World Bank's definition, China was:
low-income from 87-98 (except 97, when it was lower-middle)
lower-middle from 99-09
upper-middle thereafter
There are 56 upper-middle-income countries right now, out of 218 countries. Brazil, Kazakhstan, Libya, Malaysia, Cuba, Mexico are all upper-middle-income countries by the World Bank's latest definition.
A simple answer may be aggregates matter when it comes to country-to-country comparisons. A country's companies and government get 'strength' from the aggregate first.
I simply mean if you are comparing countries, you need to look at aggregate figures, such as total GDP or market size or population. If one country has lower GDP than another, you could say it is "poor" on that basis, even if per capita GDP (a dis-aggregated figure) were higher.
Taking your example, simply by population size and aggregate GDP and therefore ability to spend on military, India and China ARE stronger than Singapore.