The way I see it, the width of the plane is divided into three areas: seat, armrest and aisle. All three of which are wider in F than Y. Layouts vary from plane to plane, but overall I think the division is pretty much ideal. I don't feel the seats are too narrow, I enjoy the extra-wide armrest and a somewhat bigger aisle is nicer.

I suppose another factor might be that making either the seat or armrest wider would add weight to the plane. So once the seat and armrest is sufficiently wide, it makes sense to have a wider aisle, as it improves the cabin while costing nothing.