Global Share of GDP in % from AD 1 to 2000
Source: http://www.ggdc.net/maddison/oriindex.htm; Aswath Damodaran
This image shows each countries' share of GDP going back to AD 1.
Notice the United States wasn't around in AD 1 but had the largest share of global GDP in 2000 which was the most recent year on the image.
Following the U.S. was Western Europe with the 2nd highest global share of GDP.
Going back to AD 1, India and China saw the biggest decrease in their share of GDP with their declines starting around 1820.
My guess is their declines could be attributed to both countries not implementing industrialization into their economies but both countries have made lots of progress since 2000 with both their economies doing much better.