27 years old, with two kids. What causes breast size to suddenly decrease?

Hello,Yes, you have described a common concern for many women.  This is a phenomenon often referred to as post-partum, or  " Involutional" changes to the breast tissue.  In lay terms, the breasts seem to deflate.  This is suspected to be due to the profound physiologic changes that take place with pregnancy, as well as breast feeding.  It also is affected by the individuals genetic programming.Unfortunately, we know of no way to avoid such a development.  In terms of correction, this may require a breast augmentation, a breast lift, or a combination of both.I hope this helped.Thank you, S.P. Maggi, MD, FACSAustin Plastic Surgery Center 

Under the effects of hormones during pregnancy, your breast gland enlarges and the skin stretches. After pregnancy the gland reduces dramatically but the skin does not. When the patient is small breasted then this presents as lack of fullness in the upper pole of the breasts. This can be corrected with a breast augmentation. Goodluck

It depends on how far you are from your last pregnancy.  In the first year or 2 after pregnancy the breast gland shrinks and the breast becomes deflated and often sags.  Another cause is weight loss.  Significant weight loss will cause the breast to become smaller.