I also use Django for very small/lightweight/simple websites. Django is very good about staying out of your way if you don't want to make use of its built-in magic. If you ignore the "batteries included" parts of Django (ORM, class based views, forms, auth, caching, etc), it's still a good/easy/structured way of routing URLs to a python function, which seems to be the appeal of Flask. The function takes in a request object, and returns a string for output. Can't really be more simple than that.

I've usually found that more opinionated frameworks are better at making lightweight and simple applications. When working in Python, I will only reach for Flask or Bottle when I know that my problem is complex enough that Django will get in the way.