What exercises can you do to work your pec muscles and make your breasts look bigger?

Big muscle not breas. Exercises will only increase the size of the pectoralis muscle which will negligibly increase the size of your breasts. The breast is composed of fatty tissue and exercise may actually decrease body fat and breast size. Non surgical breast augmentation options are discussed elsewhere here: medications, external expanders, garments, herbs, etc. But usually reliably accomplished with implants/fat.
Not really. Female bodybuilders often have well developed pec muscles, and little breast tissue, unless they've had implants. Pec exercises make your pec's bigger, and can give the illusion of a little fullness in the upper part of the breast, but don't do much in the lower, or lateral part of the breast. Breast augmentation surgery is still the best method for this.