Why the difference and is there any way of comparing US and Euro
resorts on size ?

Divide the area by the average piste width and you're done.

You're dividing values with different units. You can't do that! At least,
if you do, you won't get a meaningful answer.

No way!
If you divide an area (m^2) with a width (m) you get a length (m). Meter is
quite meaningfull here on this side of the Nordsea ;-))

Marinus