I actually had a scene do this one to me, too... it was a complete puzzle to me.

Still is, actually - and I never managed to get it rendered large.  I think a large part of it, strangely, is the atmosphere and how it interacts with plants, water, and other objects.  If you think of it this way... for every pixel you see in a final image, the computer has calculated out THOUSANDS (if not more) of "light rays" to get the most accurate results... the more you boost your quality, or increase settings, the more calculations are required.  The larger the output, the more calculations are required as well.  You managed to give your computer a double "whammy" - as did I that day LOL.

Try using the Gbuffer and rendering to disc.  Try using the buffer to separate out objects seen in final render until you have everything you need and can composite them in photoshop or any other favorite editing software that allows multiple layers.... it might be a work-around for this.

Good luck =D
~meli

Edit:  thought of something... if you are using dynamic population ecosystem and/or a procedural terrain, THAT could be the issue as well....  many MANY more calculations are needed for the higher quality.

Rarer than a hairy egg and madder than a box of frogs....

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