It IS a pain.  Go to Material room, Advanced tab.

Select Background. Look for the lil check box that says Autofit and click it.

And then pray is religious, because THAT does not always work.  I do that all the time and still end up having to export as a PNG and put the BG in postwork, sometimes.  There are probably several things that cause the lateral shrinkage during renders, but I only kno a couple. 

The ones I know of will be fixed by Autofit.

About the others...........arrrrrrrrrrggggg.  T__T

I cannot save the world. Only my little piece of it. If we all act together, we can save the world.--Nelson Mandela
An  inconsistent hobgoblin is the fool of little minds
Taking "Just do it" to a whole new level!