Is Python good enough for big applications?
There two very important factors with python:
1) Easy to use 
2) Easy to interface with other programming languages 
FACTOR 1
Because python a language that emphasizes , simplicity , readability of code both for its own syntax and libraries, that equal to writing less code. And for software that is big this is very important. Your is simple, easier to understand by someone else, easier to debug , easier to recode , rewrite and modify. Easy to exchange between a team of many people.  
FACTOR 2
Why reinvent that wheel ? You want to use a C/C++ library ? How about a Java or .NET library ? Python will let you do exactly that and will even spoil you with allowing you to enjoy the experience without ever leaving python's syntax comfort zone. Jython, Ironpython, Cython, ctypes ,pyrex etc are excellent tools making python limitless with its capabilities let you code in all those diffirent languages/frameworks/runtimes always with python syntax. What more could you ask for ? 
In the end its python flexibility that makes it accelerate in the gain of its popularity, always hand in hand with the ease of use . Power and ease of use is a huge temptation  too hard to resist for small,big or huge developers alike.
From the moment I have faced Python, the only thing I can say for it is "It is awesome". I am using Django framework and I am amazed by how quick things happen and how developer friendly this language is. But from many sides I hear that Python is a scripting language, and very useful for small things, experiments etc.
So the question is can a big and heavy loaded application be built in Python (and django)? As I mainly focus on web development, examples of such applications could be Stack Overflow, Facebook, Amazon etc.

P.S. According to many of the answers maybe I have to rephrase the question. There are several big applications working with Python (the best example is You Tube) so it can handle them but why then it is not so popular for large projects as (for example) Java, C++ and .NET?