Where did you get the max payload figures for the JH-7 and J-16?The J-16 kept Shenyang's factories busy producing aircraft. And the J-16 is way more capable than the aircraft it is replacing, the JH-7, there is just no contest there. The JH-7 could carry 14 tons usable payload vs 18 tons for the J-16. That is like 28% more payload. The J-16 also has better top speed, range, etc. There are also other factors like it having an engine that is more similar to the one in the J-10 and early J-20s. Which vastly simplifies logistics and training of mechanics.
The H-6K/J/Ns can carry upwards of 12 (some claimed 15) tons at most.