To answer your question directly here are some non-surgical ways to gain large breasts.
Things you can not control -
Be born female to a family with chesty women. However this is not a guarantee that you will inherit this trait.
Things you can do without surgery that can boost the bust -
Visual change without physical change:
Have good posture
Wear a proper fitting bra
Wear a push-up bra
Wear a padded bra
Wear a augmenting bra with revealing clothing, bathing suit, etc.
Wear augmenting clothing with body make-up
Minor Physical Changes:
Physical changes that are minor, fleeting, or variable per individual:
Exertion that increases blood flow to the chest
Sexual excitement
Inflamation caused by allergy or dermatological reactions
Moderate Physical Changes:
Physical changes that are modest, have duration, or are consistent:
Exercise and bulking of pectoral muscles
Gaining weight
Hormones
Pregnancy
Lactation
All of these have differing degrees of effectiveness for any one person. What works well for one may have little effect on another. Another item that may or may not be applicable comes from my own reading about natural female breast development. At least three sources have mentioned that women’s breasts attain their largest volume around the age of 30, regardless of pregnancy.
So, give it some time, think about these 14 options - there must be others...