When using Print/Preview screen and the "Fit to" option is selected (e.g. Fit to 1 page wide by 1 page high), the output is stretched or condensed individually in the x and y directions to fill the extents of the page. The output does not keep the original aspect ration.
The entire output should be stretched/condensed to fit in the required pages but the aspect ratio should be maintained so that the fonts and symbols are in proportion.
I'm using ProjectLibre on OS X 10.7.5 (but I don't think it is relevant).
A nice addition would be able to specify Fit To Width (e.g. 1 page) and have as many pages high as needed.
The UI could be changed to select Fit To Width, Fit To Height or Fit to Width/Height (e.g. via drop down list).
A quicker/dirtier hack would be to use the existing UI, and allow a width or height value of zero to indicate an unspecified value => as many pages as needed.
Only "Scale to" has a constrain proportions option. You can you this option to adjust the page number. But yes it would be good to have this option for "Fit to" also