Buddhism does not need big structures as it is a theology not a doctrine like other religions.
But biggest shrine complex in the world is found in Borobudur is found in Indonesia which is world's largest(by populaiton) Muslim country.
Largest religious complex is Ankor Wat-Cambodia this shows the spread of religion to far east . All this is now a history due to religious dominance by other religions through wars.
China is doing the same as they consider religion as obstacle for their authoritarian regime.