Edit: I just thought of something else. Fi seems to be very good at understanding others' feelings, so it contains a very "watery" quality because it flows into others' perspectives. This adds some amount of abstraction. But once again it is along a continuum of other people vs. me, and it all relates back again to the self; so I think overall it still contains fewer dimensions than Ni (Ni being more than three I suppose). But it's true that they have a lot of similarities. Ultimately I think a judging function has to be less abstract than a perceiving function, because it's always going to organize things along a continuum, where a perceiving function will only go so far as to relate things to each other.