Emperor Kangxi, especially towards the latter part of his reign, firmly believed that China was in a secure economic position and the treasury was adequate. To stimulate economic activity, he froze all head taxes, (tax based on population count). This was also done because he wanted a more accurate census from local officials, who tended to under report in order to pay less taxes. By freezing this head tax, did Kangxi unknowingly trigger an unsustainable population growth? Or did he simply get a better head count without further consequences? Were there any significant consequences to this action?
The premise sounds unlikely, unless Chinese people of the period had either effective birth control (of what form?) or were killing children after birth, e.g. by exposure. Otherwise population growth is much more frequently limited by infant mortality, war, disease, availability of food etc etc.
People will keep having sex, whatever you do; unless there's some other way to deal with the consequences, babies will keep resulting from it. Don't see any way round that really.
I dug up a population graph for China, and actually the timing is not as bad for that theory as you might think.
(full-size image at the link)
If you follow the link to examine the graph's data more closely, it looks like the inflection point happened sometime in the century between 1650 and 1750. Kangxi reigned from 1661 to 1722.
However, there doesn't seem to be any historical consensus on what caused this growth. Lots of other things were changing in China then. There was a great migration of people from the North to the South. The Qing drastically reformed the tax system to lessen local corruption. The wealth of the country greatly expanded. I even found one source that mentioned that the Chinese developed a new variety of wheat around 1650 that allowed them to start double-cropping.
My own personal opinion on the matter is that changes in the rate of population growth are generally driven by changes in the marginal benifit to a fertile couple of having another child. If you aren't a farmer, and you live somwehere that discourages child labor, then more kids would just be a drag on the family. If you are a farmer, but your land is already giving all it can, then more kids would just be a drag on the family. However, if some innovation allows you to get better yeilds if you can put in more work on it than you are currently capable of doing (eg: fertilizing, pesticide application, double-cropping, etc), then suddendly an extra set of hands could make the family much better off. This is why currently underdeveloped agricultural societies are having a huge population boom, while industrialized countries now have declining populations.
So if I were to look somewhere for an explanation of the Qing population explosion, it would be Chinese farms.
NORMALLY, cutting taxes is a good measure that stimulates the economy. (Ask any American "supply side economist.) One of the side effects is that it stimulates population growth. That's also usually a good thing.
Unfortunately, China seemed to suffer from these effects, because it fell into the "Malthusian trap" population growing faster than resources. Thomas Malthus' grim vision for England was "defeated" by the industrial revolution. But it had some application to China, which did NOT enjoy an industrial revolution in the 18th and 19th century, but rather much later.