skeet
I am sure Skeet will jump on this post, the most important factor for weight is the front block,
The pro has one the rest dont hence they are much lighter, the internals as said are the next biggest thing, they all do the job but the facotry and nexus are designed to optimise this good job. Hence faster cycling better consistancy, and efficiency.
Another car analogy
VW, Seat and Audi all use the same parts, to make a good car but in the latter they are more refined and more expensive, they all do the job you want them too.
And if you are a show off you get the Audi/Nexus
A mega show off would have an RS6/DC2