Big breasts are larger than medium breasts and smaller than huge breasts. In other words, breast volume is more than the volume of a sphere with the diameter of the character's facial area** but less than the volume of a sphere with the diameter of the character's head.

Lower Bound: The breast volume is greater than the volume of a sphere with the diameter of the character's facial area
Upper Bound: The breast volume is less than the volume of a sphere with the diameter of the character's head

**NSFW Facial Area Clarification Example: link

NSFW Example 1
NSFW Example 2
NSFW Example 3

Side Chart: In construction
Frontal Chart: In construction

IMPORTANT RULES: Characters breast should be evaluated with character in the standing position of the normal portrait picture (possibly a naked one) since close up and strange camera point of view can enlarge or reduce breast size perspective.
In case of borderline cases, always prefer to tag the character with the bigger size.
In case of enlarged breasts due to grotesque breasts/body modification just use: Grotesque Body Modification