Xinjiang‘s GDP is bigger than Kazakhstan, per capita wise kazakhstan is indeed still higher than Xinjiang due to it vast resources and less population, but the difference is not very big as you claimed, ’ $9,812. vs $8484, Kazakhstan's per capita GDP is lower than China's but still higher than Xinjiang, but Xinjiang's growth is much faster than Kazakhstan, it'll overtake per capita wise too very soon.
Having a little bit higher of per capita GDP doesn't mean this region is more developed, XInjiang in general is still more developed than Kazakhstan in almost all categories, and is undisputed economic center in central Asia.