Yes,it seems logical.It is probably only an estimate,about cup size of the wearers on average,but can depends on each people.
And My-Steel seems to have updated their website some months ago to take it into account,and not only the cup size(they only asked for cup size,before,if i remember correctly).
Angelina i just think that the chastity bra would not be tight enough to be secure, she also wears undershirts because she does not need a bra
I think they took care of women with small breasts or almost no breasts,when it was designed.There is some women who never have much breasts(if I remember well what I've heard,it is the case of the actress Jane Birkin,for example,or,more recently,Maisie Williams),so it was probably taken into account.
Maybe because a classical bra is not supposed to prevent the wearer to have any access at her breasts...I think it impacts it,at least partly.