Your breasts increased in size with pregnancy as they prepared to make milk for your baby.  If you breastfed, your breasts went up and down in size daily as they made milk and your baby nursed.  Once you're done having babies, your breasts don't need all the milk-making equipment anymore, so they go through a processed called involution, where the breast tissue shrinks.  Unfortunately, the skin often doesn't shrink along with the breast tissue, so you are left with a lot of skin and not a lot of breast tissue.  This gives the appearance of "socks with sand" because the bag is still there but the contents are gone.  

Depending how much breast volume you have left, you may need a breast lift (mastopexy), breast lift with implants (augmentation/mastopexy), or just implants with no lift (augmentation).  Which operation you are a candidate for depends on what your breast look like now.

Involution is to blame for this one.  Involution is the process that your breasts go through when they are done making milk for babies.  The milk-making equipment (sacs and ducts) empties and shrinks down so you actually have less breast tissue than you did before kids.  

Depending on whether your breasts got smaller, saggier, or both, you may need implants (augmentation), breast lift (mastopexy), or both (augmentation/ mastopexy)

First, be assured that there is no such thing as perfect symmetry when it comes to breasts.  Plastic surgeons will sometimes say "they're sisters, not twins" and that is completely true. Every woman is asymmetric, though some are much more asymmetric than others.  

Sometimes one breast does all the work during breastfeeding, and that makes your asymmetry worse after babies.  This can be addressed by reducing or lifting the larger side (breast reduction or mastopexy), or making the smaller side bigger (augmentation).  It is possible that you may need a combination of these operations on each breast to get better symmetry.  

It is very common to have an increase in the size of your nipple-areolar complex after pregnancy.  Making your areolas smaller is relatively straightforward with an areolar reduction.  

Most women will not have big nipples in isolation; instead, the whole breast will be larger or saggier than it was before pregnancy.  Reducing the size of the nipple-areolar complex to an anatomic size (typically 38 mm or 42 mm) and making the areola round again are very standard parts of any breast operation (reduction/ mastopexy, or augmentation/ mastopexy).   

Your skin is made up of layers.  When your breasts grow too fast for the skin to keep up, you get areas where the deep layer (dermis) gets thin.  These are called stretch marks, and they represent lines of thinned dermis.

If you are early in your post-partum period and the stretch marks are red and wide, you can expect them to get lighter in color and fade to white, silver, or purple over time.  They also may get more narrow over time.  You have to wait until you are about a year out from breastfeeding and at your goal weight before your stretch marks stabilize. 

Once your stretch marks stabilize, if they are still purple in color, you can take some of the color out with laser treatments.  If the stretch marks are on the lower part of your breast and you are a candidate for a breast lift or reduction, they can be removed at the time of that surgery.  If the stretch marks are on the upper part of your breast, there isn't a surgical solution for them. In that case, they will be your badge of honor for your hard-working breasts.