Why not bigger? It is Thor and Kurse after all and what do you mean by more brutal?
Speaking on the theoretical level-having not seen the film or read the script (if you have it, e-mail it to me!-I would prefer not to have Surtur as the extra villain in the film. Malekith and Kurse are meaty enough to take up on the two and a half hours in the film; if C. Lee's hunches are correct, we might have Thanos' baddies appearing in the film, adding extra padding to the film. I would Surtur's presence be minimal and have him saved for Thor 3.