I have not heard this saying before. I think it was invented by the people who are selling the decoration with this saying on it.
Buit it is a "Halloween" variation of another saying -- one that is very old and common in English:
"If you can't stand the heat, stay out of the kitchen."
This saying is often used in business or the military. Metaphorically "the heat" means the strong "emotional pressure" that exists in jobs that are highly competitive, or involve physical danger, or are physically exhausting. Many people cannot cope with that much emotional pressure every day. This saying advises them to stay out of that situation (do not take jobs like that).
In those jobs, this saying is often used to "put down" (criticize and insult) anyone who complains about the pressure.
Sometimes that criticism uses the term "the big boys" to mean "men who are tough enough to handle the pressure". In this Halloween version, that has become "big girls". One traditional Halloween image is (female) witches stirring a cauldron (a huge cooking pot) filled with some kind of magical liquid.