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Solution First:
- Select your images in finder
- hit space
- manually (mouse) resize the window a little (this is crucial, as it toggles the desired behaviour)
- now enter full screen
- Bam, all images are scaled to fit the screen now.

So yeah, it's irritating...annoying....frustratingand and aggravating.
The quicklook really is a great tool. Quick and available. I dealt with it not sizing the images to the window size for so long...and now took half an hour to figure it out. I was about to give up...and just stumbled over the solution.
My problem was, I used my window manager tool, to full screen the window. This DOESN'T toggle the scale to fit option somehow...so I was just AGHH.
By using the mouse first, just sizing the window a tad bit...then using the window manager it now works. Gosh!
 
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