Some people think that a C cup is on the smaller side. The answer is more complex than you may think. A bra size is made up of a number and a letter. The number refers to the size of the ribcage, and the letter to the size of the breasts. The cup size is calculated by measuring around the bust and just below it, and working out how many inches of difference are between them.
I think it depends on the reasoning. Some people honestly just want their relationships kept private. And that's more than fine.