Zurich can draw from around 1.6M people, which is its metropolitan area with >8.3% people commuting to the center. So yeah, >4M more people is probably underselling how small Zurich is in comparison to London and SF. Nevertheless, it's not a world capital, but with its concentration of highly qualified students and graduates from two of the top universities I think it is quite competitive. Plus it helps that it's not 1.6M people isolated somewhere in a desert, rather it's a continuation of Europe's most densely populated stretch of land (London-Netherlands-Ruhr-SW-Germany-Milano), which helps at getting lots of business/talent in neighboring countries.