What Was Dr. Jekll's Theory On Hydes Different Size?

Check out the quote from the story,

I must here speak by theory alone, saying not that which I know, but that which I suppose to be most probable. The evil side of my nature, to which I had now transferred the stamping efficacy, was less robust and less developed than the good which I had just deposed. Again, in the course of my life, which had been, after all, nine-tenths a life of effort, virtue and control, it had been much less exercised and much less exhausted. And hence, as I think, it came about that Edward Hyde was so much smaller, slighter, and younger than Henry Jekyll. Even as good shone upon the countenance of the one, evil was written broadly and plainly on the face of the other. Evil besides (which I must still believe to be the lethal side of man) had left on that body an imprint of deformity and decay.