Well, it's never been determined whether Dreamstone has anything to do with the humans' evolution in the first place. And it had to be the prehistoric humans who evolved from the FF, because if not then we'd have to throw Marle's geneology out the window.
I know but, I was just pointing out that we don't know what "original size" the face is speaking about. The span at which the brain evolution process occured is 3 million years, but we don't know if the face (or maybe just the Dragons, actually) is counting only the evolution provoked by the Frozen Flame, or the complete human evolution (from the first natural human to the modern human being).