The size of the breast will "increase" or "decrease" is based on various changes in a woman's life - eg hormonal
(when pregnant), age (over time ligaments will stretch changing shape and "size" of the breast), or weight (more fat gain can increase size, weight loss
or getting lean can actually decrease them some).
There is no oil,cream,herb or anything which increses the size of breasts.
The only way to affect them in a permanent way outside the above is through surgery/implants which carries its own issues both physically and mentally. It also permanently affects the look of woman which is especially predominant as she is to grow older. Not something to be done without significant amount of forethought and personal contemplation.