Dr. Jekyll realizes that the human body is more malleable than most people recognize, more like a "mist." He therefore concocts a potion that can rearrange his body so that, as he puts it, the "lower elements" of his "soul" can become dominant.
After he drinks this potion, he feels pain, nausea, and a "horror of the spirit." He alludes to this as birth "agonies." When he recovers, he feels like he has gotten over an illness.
At this point, he begins to feel elated. He feels lighter, younger, and more physically "happy," and he feels sensuality running through him. He experiences recklessness, knows he is now far more capable of evil than he was, and yet he is excited by the prospect. He feels freed from former "bonds of obligation"—in other words, freed from conscience and superego.
He also quickly realizes he is smaller. He has shrunken, he surmises, because the evil him is less developed than the good.
The transformation shows both his awareness that he has become evil as Mr. Hyde and the seductive allure of this change.