China is the biggest country in the world, and has the most resources.
They also have a very different set of attitudes to us, in terms of training, etc.
If you are cherry picked to be a swimmer, at the age of five, and sent to a training camp to work at your swimming 8 hours a day for the next ten years, with the best facilities around you, and no distractions (like school, freinds, family life, etc), chances are you'll get quite good.
Then consider that even then, there will be 200 of these hand picked swimmers in each age group, and only the best handful will be the ones we will see on the world stage.
This is all well and good, but do you think it explains this girl suddenly, out of nowhere, swimming five seconds faster than she's ever done before, beating the world record by a second in the process and swimming a faster last 50 than the fastest male swimmer in the world?