Technically speaking fill power is a measurement of the amount of space one (1) ounce of down (as shown in the cylinders below) will occupy in cubic inches when allowed to reach its maximum loft.http://www.downandfeathercompany.com/ho ... power.aspx
For example, one (1) ounce of 800 fill power goose down will loft to 800 cubic inches. The higher the fill power the larger the down cluster. Larger down clusters will loft higher; sleep softer, last longer and sleep warmer.
The 3D structure of the down clusters creates "loft" that traps air. The greater the loft, the warmer the insulation. "Fill power" is a measurement of that loft. This is accomplished by measuring how many cubic inches an ounce of down displaces when allowed to expand to its fullest. If an ounce of down takes up 500 cubic inches of space, then it has a 500 fill power; 700 cubic inches equals 700 fill power; and so on.http://www.ems.com/ea-understanding-dow ... ation.html
That's a good part of the answer.