Letterphobia and Reverse letterphobia are terms used when someone refuses or is scared to wear a bra in bigger (or smaller for the reverse case) cups.
Comes from the fear of bigger letters bigger cup size. Normally denial that someone's breasts cannot possibly be that big, usually from a misunderstanding on how bra sizing works and still believing that DD cups are huge.
The fear to having to wear a bigger bra. Usually about bigger cups. Fear of bigger cups.
It is a common reaction after being properly fitted and finding that your cup size is much bigger than you were used to.
Thinking that you need to wear bigger cups that you really need.
This is less often seen, but some people will wear bras that are strictly speaking too big for them and sometimes confusing them with more coverage bras or thinking that other sizes won't fit them.
The term "letterphobia" is a direct translation of the Polish term "fobia literowa", which originated in the Polish brafitting forum Lobby Biuściastych and has since also been adopted in the German brafitting forum Busenfreundinnen.
When you're new to brafitting, the sizes which are common around here might seem really weird to you.