A breastcel is a femcel who has completely flat breasts and as a result becomes nearcel. The breastcel doesn't merely have small breasts; she's completely flat, i.e. not even an A cup. Some mancels however doubt that notion that women can be nearcel, claiming that there's way too many thirsty male inceldom-experiencers around who would drop their standards and link with a breastcel, even if she's an A-cupper. Juggernaut law should even predict that flat-chested women receive extra attention as many men believe they are an easy target. Moreover, a flat breast is a neoteneous feature and many males are actually drawn to neoteny.
Women who are breastcel and also have low self-esteem, often have social anxiety when they are breastmogged by women who have normal breasts. When a breastcel comes across DD-cupper however, she may damn-near have a panic attack.
The largest online community about small boobs is /r/smallboobproblems.