I'm 25 years old. How can I increase my breast size?

There is no safe/effective medication, cream, herbs, or nonsurgical  method to significantly increase breast size. Be careful about believing heavily marketed “miracles" that you will undoubtedly read/hear about.   Best wishes.

The only safe and reliable method of increasing your breast size are FDA and Health Canada approved breast implants inserted by a board certfied plastic surgeon

Breasts can be increased in size by implants or use of the BRAVA device if you have adequate stores of fat and aren't asking for more than a cup increase in size.  As for gaining weight, if you're eating well and still not gaining weight, consider seeing your local doctor to try to find the answers as to why you cannot gain weight.