"May be do you have some additional info about weight/size dynamics of Alpacas from the beginning to the present day?"
A good question. I don't have direct access to enough boats to provide solid numbers, but it's easy to speculate.
The original Alpacka, sans skirt and seat and with a small stern, was a bit under 4 pounds. Not much more than my 2013 Scout, which has smaller main tubes but a large stern. The first attached decks were heavier (70D?) sil, and thus I would assume a bit lighter than the current cruiser deck, with its heavier fabric, zippers, and lap pillow. The 2010 boats got a substantially larger stern, and then again the first pointy boats in 2011. Neither of these added much weight, but they did add some.
So how did a conventional boat go from 4 pounds to 6 in the last ten years? The deck and seat/backrest account for most of it, with smaller amounts going to the added fabric in the boat, and perhaps extra tie downs and thigh straps.
I bought the aforementioned Scout through the end of the year sale to test just what the limitations of the small, deckless boat would be under serious conditions. With four tie downs added (three for the pack, one on the floor for a seat) it weighs 3 lb 11 oz, and packs to about half the size of my 2010 Yak w/ deck and wilderness thigh straps (1.5" poly webbing). I haven't had it on the water yet (it's all frozen), but I imagine an article will come out of this experiment.