I think its more our concentration on the object being viewed in smaller FOV that brings more contrast. I have TeleVue 8mm Plossl and 17mm Nagler with 2x Barlow that gives me around 8.5mm. First with 8mm Plossl I thought I saw more contrast, but when swapped with Barlowed 17mm Nagler I saw exactly the same if not more. The only difference is that with bigger FOV you tend to look at everything there is there and not concentrate on one thing. Its harder to concentrate on particular thing when other things around it destruct you.
Something like looking at Horse neb. if you try to look at it directly first time you will not see it. But with averted vision you will, and than even looking directly at it its visible
