|Quoting embrider (Thread starter):|
How does this "weight loss" happen? What parts/component loose weight and why this can not happen in the first frames?
Mostly redesign of structural components and cleaning up systems. That takes time and data, both of which you don't have when you build the first frames. You may discover that this rib is 2% stronger than it needs to be and you can take this material away, or that you put three fans in this cooling system but it's performance is better than you thought and you only really need two.
In some cases, there may also be provisions for system modifications that you think you might need (e.g. Rolls Royce originally had a clutch between two of the spools on the Trent 1000 but it turned out it wasn't required). Once you have the data to know if you need that system or not, you can get rid of the system and the provisions that were supporting the option.
There's no inherent technical reason you can't refit earlier frames with weight-saving modifications after they're built but, in the case of big structural changes, it's economically prohibitive.
As much as you can get away with...weight is everything.