The Tibialis Anterior? (The muscle on the shin, that opposes the calf.)
We know that the central nervous system will inhibit agonist growth if a large imbalance is created between the agonist and the antagonist. I.E.-You can only get your biceps so big without training the triceps before the CNS stops growth in the biceps because it runs the danger of damage occuring from the imbalance casued by only training one of the pair of opposing muscles.
It would stand to reason, then, that you can only illicit so much growth from the calves before the CNS stops growth because we never trained the tibialis anterior to keep up. There's already a large imbalance to begin with, so making it larger is sure to cause inhinbition.
Ways to train the tibialis:
Heel Stands: Stand on your heels for as long as you can.
Heel Stands or Seated: Stand on your heels with a plate resting on top of your toes. Or do it seated.
There are a couple companies out there that make a plate loaded shoe type thing.
There's my hypothesis.