You made this claim before:
However, there are some serious issues with this. As noted before, we seem to have more than 600 million Charismatics/Pentecostals. China has about 1.4 billion people. For them to top that, China would need nearly 50% of its population to be Protestant (remember, we're not counting Catholics or Orthodox). And while I would tend to agree with you the number of Christians is underreported, it is utterly absurd that
half of the Chinese population is secretly Protestant (which is addition to any Catholics or Orthodox).
Even if we were to eschew categories and talk specifically about
organized denominations, that would require there to be more Chinese Christians than the Eastern Orthodox. My previous link put the number of Orthodox at about 291 million, but of course for a proper comparison we would need the number of
Eastern Orthodox and to not count the Oriental Orthodox. Looking at several sources, it looks like the number of Eastern Orthodox would be at about 200 million. I haven't seen any estimates of Protestants in China--again, we exclude Catholics and Orthodox--as being that high (and yes, I am sure the numbers are underreported, but I'm not talking about surveys here, but rather estimates which usually go higher). One of the higher estimates seems to be Asia Harvest, which at
Anhui - Asia Harvest puts the estimate of non-Catholic Christians at 109 million, which falls well short of the Eastern Orthodox, though it does beat out every
organized denomination outside of Catholicism and Eastern Orthodoxy.
But that is comparing two different things, because the Chinese Church isn't all one big organized group. It isn't really possible to get a good read on how they shake out in terms of ethos (e.g. how many would fit with Anglicanism, Reformed, Lutheranism, Methodism, Baptist, Pentecostalism, etc.), but it's unlikely that they're all so different from the other standard Protestant traditions that they'd all be put into their own category. Most likely they're split among all of the different Protestant traditions, which would mean none of them are likely to get an inordinate push.
Causing further problems is the fact you group them as a Protestant group completely separate from the rest. While it might not be possible to get a good grasp on how the various non-Catholic Chinese Christians shake out in terms of ethos, it is highly doubtful to me that they are so different from the rest of Protestantism you cannot classify at least a significant portion of them among the more common Protestant groups (Lutheran, Reformed, Pentecostal, Baptist, Methodist, Anglican, etc.)
So I don't think it makes much sense to say the Chinese Church is the biggest after Catholics. The higher estimates don't beat the Eastern Orthodox, and the only way to get those high estimates higher than any other Protestant group is to unfairly compare them; it might be enough to beat out
organized Protestant groups, but the Chinese Church (in its entirety) isn't organized either! Even if we suppose that every Chinese Protestant belongs to the same ethos and it's so diametrically opposed to every other notable ethos that we can't fold it into them, there's
still other ones it doesn't top.