@Salfaro Unless you eat your Big Mac like a savage, you don't actually need to open your mouth fully when eating. However, a dentist will want you to open your mouth completely and then stretch your jaw like a snake some 25% extra on top of it.
@Kaarme Sure, she'll open her mouth wider at the dentist, but my point is just that he's obviously seen her mouth a lot wider than a dot unless she drinks her meals through a straw.
Reminds me of that one Komi-san wa Komyushou desu chapter where Komi actually wonders why she sometimes sprouts cat ears, which means that she either literally does that or she can somehow perceive fourth wall-related phenomena to some extent.