Could you provide more context about where you heard this phrase or what situation it was used in? Knowing more could help me figure out what it might mean.
For example:
* Was it used literally? Perhaps someone was talking about a large pastry bun.
* Was it used metaphorically? Maybe it was referring to something else, like a large sum of money or a significant problem.
* What was the tone of the speaker? Was it serious, humorous, or sarcastic?
With more information, I can give you a more accurate interpretation of "a big bun."