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.

Sister size, Bustyresources Wiki

What do Bra Sizes Mean? – Curvy Kate CA

Tips for Ordering a Wire Bra

Should I go up or down in sister sizes? I'm wearing a 42D but the underwires on my chest are poking my breast from the wire ends and its uncomfortable. What sister

When a women loses weight does her bra cup size change? Why? - Quora

Bra Sister Sizes + Sister Size Chart

Bra size - Wikipedia

How Bra Sizes Work: A Full Guide

I'm lost! Am I a 32G/GG or a 34F? : r/ABraThatFits

Understanding Bra Sister Sizes

What do Bra Sizes Mean? – Curvy Kate US