In a traditional shrouded turbine every additional foot of tower height would require, at a minimum, 2*pi feet of additional circumference to the shroud (in the case of the design that started this discussion it appears that the shroud is the main structural element so it is a bit different, but similar principles apply.) The shroud has weight. As the shroud circumference increases it will require both a stronger tower and stronger internal supports to handle the ever-increasing weight of the shroud. It is not hard to see that as the weight of the shroud increases most of the tower and most of the structural mass of the shroud becomes dedicated to holding up the shroud itself and an ever increasing proportion of the tower/shroud mass is dedicated to holding up the extra mass that is only necessary to keep the rest of the shroud from collapsing (e.g. structural mass to hold up the structural mass that is keeping the shroud up), with an appropriate increase in the cost of the tower. The exponent is probably closer to 1.1 than to 2 or more, but exponential is the proper term here.