Here’s a post on the Worthwhile Canadian Initiative economics blog with basic data on the rise of China as a share of global GDP. Note that this is done in PPP (purchasing power parity) terms, which is good at comparing consumption but not necessarily “clout” in global markets, where market exchange rates matter. More come the new term on how to undertake (cross-section) comparisons of economies.
GDP in developing economies is prone to underestimation because the informal sector is large while GDP focuses on formal market-oriented activity.