I’ve done the research and sure enough scoring (etching a shallow line with a knife) a frozen hot dog before you grill it does in fact result in a plumper, less wrinkly hot dog. Why is this so?
Hard to figure. By and large, scoring or poking food before cooking allows for the escape of steam, which would otherwise cause swelling and…plumping. You seem to be getting the effect by doing exactly what would be done to avoid it. Curiouser and curiouser. By the way, can you describe the experiment you did so we can evaluate the controls, the procedure, the data collection, etc.
You relieve the tension that the outer surface has which resists expansion of the middle. Think of it as pleats on a pair of pants.
Aha—that explains why they deform and explode in the microwave if you don’t score them, but expand nicely if you do score them. However, what is causing the expansion itself? Other meats don’t expand when you cook them, at least not as noticeably or dramatically as do wieners.
They’re hot dogs. What does meat have to do with it?
Sadly, the plumpness of hotdogs is caused by nitrates. I’ve tried to make my own nitrate free hotdogs, but it’s extremely difficult. Hotdogs are an emulsion in which each tiny piece of ground meat is surrounded by fat. When you prepare and stuff them everything must be kept cold enough to keep the fat solid, or it separates. I’ve never been successful.