Maybe it's just how I pictured or imagine it in my head when I read the descriptions, but didn't people say there was a more humanoid version of the Lizard before the final, massive one appeared? Look for the Lizard to have some clothes in his early forms throughout the movie.
I think what they're going for, which makes sense, is Dr. Connors/The Lizard to lose all touch with humanity as the film progresses.
Ie:
He starts out as a doctor trying to develop something that is revolutionary, the experiment goes wrong but he still maintains a piece of himself, as the film and story continues, he tries relentlessly to fix his formula but only makes it worse until he eventually becomes this beast of a creature and loses all touch with what he used to be which would explain Peter's final plea in one of the footage reels shown:
"DR. CONNORS! THIS ISN'T YOU! STOP IT! STOP IT NOW!"