It could have to do with conditions. For the record, the Irish book describes the leaves of decipiens as 29-39 in. long and says "it is best to consider placement carefully to allow for the full massive size of a mature individual". Gentry agrees about the size and says "in the fertile soil of the Florida peninsula the plants are said to reach 4m in height with leaves exceeding 2m in length." Around here the plants pretty quickly become large, and with enough offsets at the base they can turn into a impenetrable thicket.
Regarding the speed of the Queen Victoria agave (and its ally, nickelsiae).... these are supposed to be very slow growers, like some other agaves (eg. "Cream Spike"). They certainly are, at least in comparison to americanas and the like. But in my experience that's only relative. Maybe that's because our growing season is year round. One plant here went from a 6" pot to a 12" pot (stepwise) over the course of 3 years, to give you some idea of the pace. There are a few different varieties of the QV agave, the main two in cultivation being a larger, solitary form (grown from seed) and a smaller, offsetting form, which will never reach the same mature size.
