Tina is drawn improbably stacked for a 13 year old, but this doesn't elicit much comment from the other characters around her. Therefore she didn't look like that when she was 13. Instead first person narrator Tina is projecting a chibified version of her current body shape back into her 13-year old adventures.
My head canon is that adult lonely sexy librarian Tina is imagining these scenes as she writes them up while waiting for anybody to ask about books.