Breast size mainly depends on genetics, hormones (especially estrogen), body weight and nutrition. At 21 years, most breast development is complete, but mild changes can still happen naturally.
There is no medicine, food or shortcut that can increase breast size quickly, but you can support natural growth by maintaining a healthy weight, eating balanced meals with good protein and healthy fats, managing stress, and doing regular chest-strengthening exercises (these improve shape, not actual breast tissue).
Next Steps
If you feel your breast growth is much lesser than expected or if you have irregular periods, acne, hair growth etc., a hormonal evaluation is helpful.