Memory leak in ActionController::Metal (3.0.7)

Consider the following class:

class Myclass < ActionController::Metal
  def myaction
    huge_object.do_something
    # etc.
  end

  private
  def huge_object
    `@obj` ||= begin
      # allocate and initialize a ~10MB object
    end
  end
end

What I was amazed to find, is that even in production, each time the myaction is called, the object is allocated. Moreover, something is holding a reference to it, as the GC doesn't collect it. I made a workaround by wrapping the object in other class and including Singleton in that class. Still, I want to understand, what's going on here.