Bustyresources Wiki

One can get quad-boob when a cup is too small.[1]

Quad-boob is when one's breasts spill over the cups, either on the tops or out the sides (in the armpit area), giving an impression of four breasts.[2] Most often, if this occurs, the cup is simply too small for one's breasts.

Possible causes[]

Small cup
When you have issues with quad-boob, the first thing to assume is that the cup is simply too small. In general, try going up in cup size (but not band size) until you don't have resolved the issue. If this still isn't working, consider that the issue had one of the following causes.
Soft breast tissue
In other cases, quad-boob can be caused by cups not suited to one's shape. Many plunge bras are too closed and cut into the breast. Some simply have too soft of breast tissue to wear plunge and half-cup bras, and these women will always get quad-boob in these bra styles, despite cups being the right size, or even too large.
Shallow breasts
For example one with a shallow shape and/or with a lot of fullness in the top of her breasts will have problems with cups that are too closed. The right size half-cup bra is a good solution.
Tight band
Also, quad-boob can be caused by too tight of a band. In a small band, it pulls the cup flat, and the rim of the cup cuts into one's breast. Insert an extender into the bra band, and see if it gets better.
Narrow underwire
With underwire too narrow, the breasts' weight may push apart the wires, causing the same problems as above. Try a bra with wider wires in the same size, or go up a cup size.