Hmmm, now I'm starting to wonder... are you seeing really tiny ones somewhere? The ones we see, as high up as we see them, are still around 10" - 12" tall in flower... ?
Calgary, Alberta, Canada - Zone 3
-30 C to +30 C (rarely!); elevation ~1130m; annual precipitation ~40 cm
That's probably about right- the smallest ones I've seen were not in flower, so who knows if they were even mature. Here, though I haven't measured (I'll take a closer look at the dried seed stalks on mine when I go back out) they must get close to 2 feet tall with large clumps of large leaves on mature plants.
EDIT- I'll post a pic of mine in Image of the Day..
west central alberta, canada; just under 1000m; record temps:min -45C/-49F;max 34C/93F; http://picasaweb.google.ca/cactuscactus http://urbanehillbillycanada.blogspot.com/