GDP estimates are by their very nature arbitrary, and can be easily manipulated. There is a major difference between GDP estimates for China being too high, and China actually having double the population. China isn’t some hermit kingdom - westerners can readily move around the vast majority of the country. Why would every international statistical agency co-operate in covering up the existence of more then a billion people?
Just what sources do you have for this?
Charlie Smith has written for quite a few publications, it is hard to keep tract where it might have ended up (Newsmax, his own site Software.net and some intelligence and business oriented intelligence reports). I will do a little digging and see if I can come up with anything. Smith is amazing with what he unearths. Check out the link to his old site. Unfortunately, I don't have is email address.
The real issue for the intelligence world came when they started doing grid counts of people in the rural areas of China. There were just too many people in grids to match with the public Chinese statistics (by a factor 50%). I haven't been able to find a source yet...and I still need to get some work done tonight.