I think the issue is with a bigger SG, when the lengthened chin pulls the lower lip downwards and to compensate for this, the lower lip tries to remain a closed mouth by rolling inwards.
That's what I've heard about lip issues and SG. Even with my own small sg(4mm forward & 4mm downwards) my lower lip rolled in a bit. For me that was a plus because I did not like how low my lower lip hung down naturally.