Building blocks / concepts
I'd be surprised if npm universe is not >10x bigger than all public c++ libraries combined.
The question is too vague to have any meaningful answer.
I think the C++ standard might be the biggest. Like, just the amount of compiler passes it does and what means when is kind of insane. I shouldn't even have seen that in my life, yet I did. I guess that's what friends are for :')
But the ecosystem of C++ seems quite small compared to others as C++ programmers want to build stuff themselves.
So it depends if you just look at the standard versus the ecosystem.