Why might it be that my boobs look smaller than my bra size measurement?

Your perception: There is a great deal of psychology involved with breast size. A woman who prefers big breasts often sees her own breasts as being too small. A woman who thinks that small breasts are better may think hers are too big. Some women just want the breast size that will please their man. There is the band measurement size & the cup size. Many women's perception of their breast size is way off.