Feature-rich vs focused libraries, which do you prefer?

I have seen a lot of libs out there, but in general I can see two directions: On the one hand, libs which are very small and focused. They do one thing, but they do it well. It's a little bit like adhering to the Unix Philosophy.

On the other hand, there are more and more big libs, which try to do more and more under the hood, basically being like a big black box you use and which does a lot for you, often even auto-magically.

What do you prefer? Why? In which situations? What do you think about prototyping using big libs, but then creating very lean applications with small libs? Would you get the time for that?