Bug? Screenshots as BG Images Don't Size as Expected

I’m wondering if this is a bug or just me. Please try this:

  1. Take a screenshot of a webpage, any webpage. Just make sure you get your entire browser port content.
  2. Paste that into a new Axure file. Hit Preview.
  3. If it’s like me, the page you now see does not size to your browser correctly. In fact, it’s rather huge.

Is this a bug or am I missing something dumb?