To be honest, when I first saw the thumbnail I mistakenly thought the proportions were not quite right; it seemed a bit blunt -- but when I looked at the larger view, it looked great, and I can see that it does indeed confom to the proportions of the skull.
I was just thrown off at first by the heavy dewlap or throat pouch. Which as far as I know is a perfectly legitimate feature to speculate that Dimetrodon had -- I'm simply not used to seeing it in restorations of this beast. Anyway, the more I look at this, the more I like it. It is a beautiful rending of one of my favorite extinct animals.