Why are faces in my model fine in the viewport but distorted in the render? I didn't find the mistake in my properties, perhaps somebody can help me. 
This is my object in the viewport ('Rendered' shading mode):

 

This is the rendered result: 



The problem are those distorted faces. It looks like the faces are getting shrunk. 

I'm using Cycles Render, HDTV 720p, Aspect Ratio 1:1... If you need further information, just ask.
It looks like you have a Subsurf modifier on your object that has been disabled for the viewport, but not the render. Either delete the modifier or disable it for rendering as well by clicking the camera icon on the modifier: