bé bằng củ khoai, cứ vai mà gọi

Etymology
Literally, "small as a sweet potato, called by his rank". In Vietnam, people are traditionally referred to (including with honorifics like "grandfather" which superficially invoke age) based on their social rank, not their age, so a low-status old man may refer to a high-status child (who is as "small as a sweet potato") as "grandfather".