Why does China have as many if not more heavy fighters than America (global commitments) and Russia (larger territory) combined.
With US - many reasons. Partially - requirements of the theater (Pacific/naval), partially lack of aerial refueling fleet, partially availability of particularly liked model (flanker), partially - lack of general expeditionary optimization.
T-10s and J-20s were a better fit for the specific historical moment, and there was only one available light type(J-10A/B/C), which was built in a somewhat different China, and as such closes only it's specific role, for which there's more than enough of them.
This moment changes, of course, now the proportion of lighter aircraft will grow through J-35 and especially LWs.
With Russia, it's more straightforward - China has x10 the economy(or x6 PPP). Russia, despite it's size, needs smaller more affordable fighters to keep up numbers - but it just doesn't have them. It results in a rather small fleet for what the country needs.