LOUISE: Tristan suddenly has very big eyes for you, Grandma.
Louise is referring to the well known European fairy tale Little Red Riding Hood, where a little girl in a red hood takes a basket of food to her grandmother in the woods, and foolishly tells a wolf where she is going. The wolf races ahead of her and devours her grandmother, dressing in her clothes to deceive the little girl, who notes, “What big eyes you have, Grandma!”.
The fairy tale is generally seen as having erotic undertones, as the wolf sweet-talks (seduces) the little girl into telling him her destination, and he also eats both the girl and the grandmother (ravages them) before they are rescued by a huntsman, who cuts them from the wolf’s belly.
We still describe a man who is sexually dangerous as a “wolf”, and it is apt that Louise slyly links Tristan with this seductive fairy tale character.
The Annotated Gilmore Girls 