I just googled "fore and aft fly pattern" and clicked on "images" in the results. It looks to be roughly 50/50 about the smaller rear hackle (of those images which were actually of fore and aft flies. It doesn't seem to matter whether the pattern has a tail or not.

I can certainly conceive of using the same hackle for both parts and having the back smaller -- tie in by the tip at the rear, wrap, put down a body over the middle of the stem, and wrap the front. Not that I would do it that way, but it could be done, and probably has been.

I think the fly looks better with equal size hackles.