You do realize that breasts are a half sphere.
2 * R exp 3 is the volume of this half sphere.
so you've got 2.1 * Radius exp 3 = volume of half sphere
Say at 3 months, your breast have a radius of 1 inch, at 12 month 2 inch, at 24 month 3 inch.
3 inch radius is about a C cup.
The breast tissue would be at 3 months = 2.1, 12 month = 16.8, 24 months = 56.7
While the 3 month to 12 month rythm keeps with the 0 to 3 month,
if you kept with the same rythm in 12-24 months, you wouldn't have 56.7 at 24 months
but... 33.6!!
So, your breast would have grown at the same
speed but not reached close to C cup even 12 month later.
The projection would only have increased 0.35 inch ((33.6/2) exp 1/3),
yet your breast will have grown just as much as before.
That would put you a bit tight in your B cup but not yet ready for C.
Since, its a much bigger envelloppe, there's also less likelyhood of tightness
in the breast so you don't feel it growing in the same way
(though, you may feel transitory pain).
The bigger the breast is, more tissue it needs to increase visual size.
