Size:
* Literally small: If referring to physical size, it simply means something is not large.
* Example: "My apartment is not big, but it's cozy."
* Moderate: Can indicate something is neither large nor small, but somewhere in between.
* Example: "The town is not big, but it's not tiny either."
Importance:
* Unimportant: Can suggest something is not significant or noteworthy.
* Example: "The mistake was not big, so I didn't worry about it."
Other:
* Not a major deal: Can be used to downplay something, implying it's not a serious matter.
* Example: "The delay was not big, just a few minutes."
To understand the meaning more accurately, you need more context. For example:
* What is being referred to as "not big?" A person, an object, a situation, etc.?
* What is the speaker's tone? Is it casual, serious, dismissive, etc.?
Please provide more context so I can give you a more precise answer.