The one for the Slim is narrower, but it isn't shorter. From socket to post, it's the same length. It's a bit misleading because if you look at the slim neck post, it actually looks longer, but that's just cosmetic. Again, from socket to post, and how it sits on ANY TrueType, they're identical in height.
You're right.
I wouldn't think so, but my last guess why Sparrow didn't get shorter must be the little difference in the height of the joints.
Anyway, for folks to see, here are 3 of the necks in question, I don't have the Tony Stark suit, but that neck is only useful with him anyway.
The slim is cut differently at the bottom to give it articulation like the other bodies at the neck, which is why a hair is lost when you use one of the regular ones, it's a more snug fit inside the shoulders.
SLIM / STANDARD - NARROW / TDK