The girls I mentioned use a 12-inch wheel (actually a Wobust Wodent Wheel, which they run 2-3 at a time in) which is more than big enough for them. I, personally don't believe that's the problem, I think it's how often they're running in the wheel. At least, that's my theory. They're the only girls with an extreme passion for wheels - running about 90% of their awake time, no kidding, and they're the only girls that have wheelie tail. I have other girls (a bit larger) that love the wheel (they use Comfort Wheels, which I believe are also 12"), but they're not in it all the time, like the 5 girls and don't have wheelie tail.
That's my thought, anyhow.