React isn't big, it's about as small as a "framework" gets. The problem most people face is actually that the simplicity gives the developer unlimited freedom to craft whatever complex monstrosity they desire.
Your point is like saying C is a lightweight language. It sure is, until you have layered everything else you need on top of it.
Saying react is complex is much like saying C is complex because of OpenGL or SDL.
React isn't the ecosystem anymore than C is.