I found this question sort of interesting, so I went to look up estimates for world population at different times in history. I know this frowned upon here, but wiki actually very conveniently has a table with different estimates from different sources. http://en.wikipedia.org/wiki/World_population_estimates While I think it's interesting how well a lot of the different estimates line up, they don't always line up, which should give a sense of how hard it is to really say with any certainty what the population of the world was for any given time up until very recently.

I then started to look up populations for various large empires that I could think of off the top of my head before I realized that Wiki actually has that already as well! http://en.wikipedia.org/wiki/List_of_largest_empires

Now, looking across all the sources cited on the wiki pages, it's easy to find some discrepancies (for instance, two very different estimates are used for the total population of the world for the Han dynasty and for the Roman empire in that second page. The first few centuries AD are an area where the population estimates don't line up very well). Even though, it seems clear that there are actually a surprising number of candidate states/empires that had a larger portion of the total world population than China currently has. That actually includes a number of earlier Chinese dynasties, various Indian empires including the Mauryan and the Mughal empires, the Aechimenad empire, yes the Mongol empire, possibly the Roman empire and possibly the British empire. Given how disparate some of the size estimates are, it's hard to say if all of those states actually were a greater percentage of the total world population of the time, but it looks a few certainly give modern day China a run for its money.

I don't think China has the biggest % of world population.

Guinness world records has the Achaemenid Empire with the largest percentage of the worlds population.

By share of population, the largest empire was the Achaemenid Empire, better known as the Persian Empire, which accounted for approximately 49.4 million of the world’s 112.4 million people in around 480 BC – an astonishing 44%. Originating in modern-day Iran, the empire was first established by Cyrus the Great and included parts of Central Asia, the Mediterranean, North Africa, and even European territories such as ancient Thrace and Macedonia.

No, I do believe that distinction goes to the Achaemenid Empire. That polity came into existence circa 550 BC, and conquered 3 cradles of civilization: Egypt, Mesopotamia, and the Indus Valley. The significance of this, is that while China is very large today, there exists population centers and civilization all throughout the world. This wasn't the case in the time of the Achaemenids, and the territories that their empire encompassed were among the few sites of civilization in all the world at the time. Even the concept of a 'city' was confined to very few places. So essentially, many of the high density areas on Earth were under their banner. Though I doubt the figures are accurate to the T, according to Wikipedia, the Achaemenid Empire was home to nearly half of the global human population at their peak-significantly higher a proportion than that of modern China.