Sometimes my breasts are size x, sometimes y

What is it about women that have different breast sizes ? Is is just that they are exactly at the boundary between two sizes, and jst when they do the measrement, sometimes the noise puts them one the one side and sometimes on the other ? Or does the size actually vary, and if yes, according to which parameters ?

 

Hormones

For me at least. They get bigger or smaller depending on where im at in the cycle. Ive heard this is common