벌리다 is pulling two sides apart, and 입을 크게 벌리다 — open your mouth wide — is six of these eight lines, all about getting food in: 턱 빠질 것 같이 입 크게 벌려야 해요 (you have to open wide enough to unhinge your jaw). It also covers spreading your legs apart, which is what 왜 소민이 앞에 두고 저렇게 벌리고 있어! is complaining about. Contrast 열다, which is opening a door or a lid.