It might be due to a couple things - if the length of the window is also short, it will not appear as long and narrow as on the new Revell kit.
I have been looking at a lot of cars for my WIP and one thing about the Fox body is the ratio on the side window - the height to width ratio - that gives it a certain look (difficult to describe in words, and probably pablum from me...) and goes with the rest of the body proportions. The current proportions on the Revell kit make it look like a 1990 Cougar, which is a different kind of car (larger, cruiser type vs small, sporty) and just throws "The Look" off.