The weight of the passengers is a very small part of the whole coach's weight itself, so increasing the capacity of the coach by changing the internal layout doesn't make a big difference for the coach's weight and speed potential as a whole. Each coach weighs less than half the weight of a standard LHB coach, and is a little over half the length of an LHB coach. The decrease in capacity per coach, can be compensated with more number of coaches. There will be some reduction compared to a standard rake, but the difference will be compensated by the reduction in operating costs and higher speed potential.
Also Talgo's higher speed capacity is primarily due to the...
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shorter coaches, less ground clearance and vestibule design for zero air gap between coaches. All these reasons mean that even with longer rakes, the coaches will still retain a higher speed limit than the LHB coaches on similar track conditions.
The most attractive feature however, is the ability to use all these advantages while spending very less on track upgrades. Signalling upgrades will still have to be made, but usually, the track upgrades and related construction activities are the ones that end up taking several years in our country. Talgo can be quickly brought to service as they are pretty much capable of meeting higher speeds on our existing tracks. If IR doesn't screw it up ( due to politics or whatever), this will be a good bargain for our system.

Why did Telgo make shorter coaches ? with reasonable idea that it will be easy to haul them than larger ones...
and this is right choice if it is size...
Not why to increase the length...
In future short size coaches will pave the way for selective augmentations of coaches depending upon the 'booking rush' ... it is as good as pressing in...
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more buses whenever passengers are more...
But the idea of 'berth weight plus passenger weight' is still too early to down play ...
As Car technology also is gradually using more of plastic and amagmated metals in place of original iron casings in body and engines ... So is with aviation and Shipping... to save fuel and increase speed...
So weight is always the problem whether it is 'own' (coache) or other's (passenger)
Still your explanation is very logical... I like ..

Thanks!
There is no particular problem with hauling larger coaches ( Most of our locos are more than capable of handling such loads) The reason for making shorter coaches is the need for making high speed coaches. You can search here on IRI for some posts explaining the reason why longer coaches of the Indian style pose problems for higher speeds. And also, in the Talgo rakes, each coach shares a bogie with the adjacent coach and there are no couplings of the kind we see in our Indian rakes. All this makes short coaches necessary and also helps them in traveling at higher speeds on curves.
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You are right about the coaches being lighter, but usually in reserved coaches where there is no overcrowding, the total weight of the passengers is only a small fraction of the coach's weight. In an average Three tier AC coach, the weight of the passengers is barely 10% of the total coach's weight, and the corresponding increase in the power required from the loco isn't much. In this scenario, the Talgo will still be efficient because of the lighter coaches. While a 3 tier AC coach with 72 passengers weighs about 50 tonnes, the same capacity is possible with 2 Talgo coaches, which together weigh about 26 tons. So overall, it is still better for the loco.
That said, not everything is rosy about the Talgo coaches. Platforms have to be modified, maintenance facilities have to be setup, rake augmentation or modification is a big hassle among several issues. But, it's a good start at least.