Refers to the belief that all bra sizes range from 32-40 bands and all cups are A to DD cups and that anything other than that does not exist. This goes hand in hand with the myth that D/DD cups are huge.

Breaking out of the bra matrix is usually associated with letterphobia as disbelief that they need a cup size bigger than DD.

Matrix sizes

The term "Matrix sizes" is used when talking about sizes in the 32-40/A-DD range. Usually what most brick and mortar stores will carry.

	A	B	C	D	DD
32	32A	32B	32C	32D	32DD
34	34A	34B	34C	34D	34DD
36	32A	36B	36C	36D	36DD
38	32A	38B	38C	38D	38DD
40	32A	40B	40C	40D	40DD

Interpretations

There are two interpretations of the term "Matrix":

Talking about the table of bra sizes, used as sizing matrices/charts of sizes.
Referencing the Matrix movie, where there is a whole world that exists that you are not aware of. The analogy would be that once you learn about all the new sizes you'd be taking the red pill, also called as "Breaking from the bra matrix"

You take the red pill, you stay in Wonderland, and I show you how deep the rabbit hole goes.

Origins

It originates on an old polish bra blog called Stanikowy Matrix where the author explains the Bra Matrix as the sizes that "are stuck in your head, same sizes your breasts are squeezes in".

This is the original blog post: Bra Matrix

The first writing in english comes from the Venusian Glow blog, a few months later she translated for us and explained her discovery on how she was stuck in matrix sizes.