Here is data from the IMF, for example, showing US and EU area GDP as a percentage of global output over time, gradually declining since 1980. The decline from 1960 to 1980 in the US was even more dramatic: in 1960 the United States GDP was about 40% of global GDP.

https://www.imf.org/external/datamapper/PPPSH@WEO/EU/CHN/USA