While the B cup size is average, the bust measurement of 36 inches is slightly larger than average. Bra sizes are unlike most other garment measurements, in that they require measurements for the circumference of the body at two separate points.
In this instance, the measurement itself consists of the circumference of the torso just below the breast at 36 inches, and the circumference at the widest point of the breast at 38 inches. The two inch difference denotes a B cup. Around the world, manufacturers have standardized sizing for cup sizes all the way up to D, or 4 inches difference between torso and the widest point of the breast. However, it is difficult to provide a quantifiable comparison between breasts as the human body varies widely from one individual to the next. In short, a woman's bra is by no means an effective way of comparing even something so trivial as her breasts to those of another women. Age, environment, health, diet, activity level and whether she has had children can all radically alter the shape and appearance of the human breast, and these are just the most prominent factors. Many other things can also affect the size, shape and appearance of the breast as well, all of which are quite beyond control.