Medium breasts are larger than small breasts and smaller than large breasts. In other words, breasts volume is more than the volume of a sphere with half the diameter of the character's facial area but less than the volume of a sphere with the diameter of the character's facial area**.

Lower Bound: The breast volume is more than the volume of a sphere with half the diameter of the character's facial area but less 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 facial area

**NSFW Facial Area Clarification Example: link
NSFW Facial Area Normal Breasts Example: link

NSFW Example 1
NSFW Example 2
NSFW Example 3

Side Chart: link - From right to left: None, Small, Medium, Large, Huge
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