“As I have stated I would rather have a 700 down sleeping bag than a 900 down sleeping bag because the 900 is going to collapse under any weight of pressure BECAUSE IT IS SO CAOLAPSEABLE/EXPANDABLE.”
It seems any down is collapsible. If you want to make it more firm, just stuff more down in—which also makes it more warm. For a sleeping bag, the down is on top; thus a tough LIGHTWEIGHT material shell won’t collapse it. It will poof up faster.
“Where 900 may be "better", what does better really mean?”
It should mean higher quality and be warmer for the same weight of 700 down (w/o overstuffing volume). I think TomD noted this sight a while back: http://www.downandfeathercompany.com/categories/How-to-Choose/How-to-Choose-Fill-Power.aspx
I like it’s pics of comparisons and ‘fill power facts’.