A 28DD is small. VERY small. Its sister sizes are 30C and 32A. A 42DD is massive. The cup is relative to the band size.

What is Sister Sizing? – Fleur of England

Question] How “off” were you before you found A Bra that Fits? I can't believe I ever thought my old bras fit ok! 🙈 : r/ABraThatFits

How to sew a bra that fits: A bra fitting guide for large busts

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Sister Sizes: The Bra Secret Every Person Should Know – Sister Sizes & Equivalent Bra Size Chart – ThirdLove

Meesa - Sister sizes are sizes that have the same cup capacity but a different band size. Find out yours here! 😉🙃 . . . . . . . . #bra #sistersizebra #onlineshopping #womenforwomen #meesa #meesa_np

Bra size - Wikipedia

Is it normal to be 13 and have DD in bras? - Quora

What is Sister Sizing? – Fleur of England

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Choose the correct bra cup size when changing band size

Sister Sizes: The Bra Secret Every Person Should Know – Sister Sizes & Equivalent Bra Size Chart – ThirdLove