Small breasts are tags attached to illustrations of women with small breasts. The antonym is big breasts.
The word small breasts has no academic (medical) definition.
Physiologically, the breast only needs to swell about one year after breastfeeding, and does not have to swell from normal times. Many mammals other than humans have flat breasts even in adults.
In general, the difference between top and under, that is, the AA cup, A cup, and B cup, are often regarded as small breasts when distinguished by cup size. Recently, as the number of E-cups and above has increased, some people treat C-cups as small breasts.
When defining by bust size, it seems that 80 is often the boundary, and when it exceeds 80, it tends not to be said to be very small breasts.
Originally, it is correct to use when it is after the end of the secondary sexual characteristic period.
This is because the majority of women before and in the middle of secondary sexual characteristics have small breast bulges.
In addition to small breasts, short stature tends to have a legal loli meaning.
The "model body shape" is basically small breasts.
The reason is that if the body has large undulations, it will be difficult to make the pattern and structure of the clothes look as the designer intended. (See Big Breasts Trouble)