My informant heard this from her Vietnamese mother as she was growing up

“Wearing a hat indoors makes you shorter”

My informant is pretty short, so she uses this as an explanation of why she is short.

Asians are, on average, shorter than Caucasians and Blacks, and height is seen as a favorable attribute.  Plus, it is impolite to wear a hat indoors, possibly because it means that you do not intend to stay or do not trust your host (you are hiding your face).  Combining those ideas creates this idea that wearing a hat, or being impolite, will have a negative effect on how you appear to other people.  My informant likes to wear stylish clothes, including very pretty hats, and uses this saying as a joking justification for why she is the way she is.  It implies that she would have been taller if she could have resisted the temptation for stylish hats.