You are operating under the assumption that people usually wear the right bra size, which is way untrue. The main thing is that people were told to add 4 inches to their band size, then each extra inch beyond that of busy measurement was a cup. So, a thin woman wearing a 34C likely had actual measurements of something like 30" around her ribcage and 37" around her breasts. Now, in actual correct sizing, she'd wear something like a 30G.
You just aren't used to it because most people, especially those that have some weird fear of wearing something bigger than D, wear the wrong size. VERY few women you'd call thin have ribcage measurements of > 32", and 28” isn't unreasonable.
Look around on that site I linked to. Those people are actually wearing the bras of the sizes they list, the numbers don't lie.