Size of market is just one possible reason. There are others like those you have suggested. China population may be large, but disposable income is small on average n hence the addressable market while large may not be big as a proportion of its size.
Did a quick look and saw that the usd/jpy and sgd/jpy seems to have been behaving about same the last 3 months ... the average goods & services tax rate in the US is about 5%+ ... so these does not seem the strong supporting reasons. If market size wise is the reason (as you said), then the pricing method are simply discouraging people from buying in this part of the world rite?