Finally going through those links today Simon. I disagree that these show what you say they show. In the one with five specimins of S. megalanthus from commercial plantations in Colombia and Ecuador, it says, "Fruit of S. megalanthus (80–300 g) are much lighter than Hylocereus spp. (200–800 g)." There is even a section, "Possible cause of low fruit mass in S. megalanthus", which suggests it may be caused by too few of the ovules developing into viable seeds (8.7% with self-polination, 9.7% with H. polyrhizus pollen, 5.4% with H. undatus pollen, and 17.9% with S. megalanthus pollen), and that the fruit flesh may develop from the viable seeds only. I don't see anywhere in here, or in any of the others you posted, that says there is actually a difference in fruit between Colombian and Ecuadorian genetics, or that either of these are different than the wild (common) form.
It really sounds to me like those researchers took cuttings from commercial plantations in South America, grew the fruit, and didn't have any fruit over 300g. I'll also add that if they just have better pollen, like some Selenicereus species that's closer to megalanthus maybe, that caused 30% or 40% of the ovules to develop, that sounds like it could cause the fruit from South America to be giant.