Is bra size 28 small

There is no such thing as a "bra size 28". Two different things go into a bra size: the band number and the cup letter. You would not say you wear a "28", in the same way you would not say you wear an "F". A cup letter means nothing without a band size attached to it, as the cup letter will change volume on different bands.

The band number is (in the United States and United Kingdom) how many inches your ribcage measures, which is where you wear your band. If your ribcage equals 28 inches, you would wear a 28 band. If your ribcage equals 33 inches, you may find firm 34 and loose 32 bands will work for you.

The cup letter means how many inches the difference between your ribcage and your bust are. If your ribcage and bust both measure 24, you would wear a 24AA, since an AA equals a 0 inch difference. If your ribcage measures 36 and your bust measures 46, you would wear a 36GG, since a GG means there is a 10 inch difference.

A 28 band is small compared to every band size 30 and higher. This does not mean only "small people" wear a 28. Women and girls in all sorts of heights, weights, body frames, and ages wear 28 bands. There is a huge market for 28 bands, which companies like Freya, Curvy Kate, and Ewa Michalak cater to.