I ruined a small one. Here are tips from the University of Nebraska Cooperative Extension on how to wash pillows.
Follow the manufacturers’ cleaning directions. If there aren’t any, use the following guidelines. Although down-filled pillows are hand or machine-washable, some manufacturers recommend dry-cleaning.
Occasionally place feather and down pillows in the dryer on low heat for 10 minutes to remove humidity and help keep them resilient and fresh. Polyester fiber-fill pillows can also be freshened in the dryer on low heat. Foam rubber pillows shouldn’t be placed in the dryer.
If dry-cleaning down, find a reliable dry-cleaner with experience in cleaning down. Afterward, thoroughly air the pillows to get rid of lingering fumes from the cleaning agent.
Pillows filled with down, feathers, or polyester fiber fill can be washed in the washer on a gentle cycle with warm water and a mild detergent. Check the cover of the pillow to be sure it’s strong and that the seams are tightly sewn.
Wash only two pillows at a time. Fill the washer, then submerge the pillows and wash four to eight minutes. Rinse in warm water three times to be sure that all detergent is removed.
Thoroughly dry pillows in the dryer at low heat on the gentle cycle. This may take three to six hours for down; less for polyester. Placing a clean tennis shoe or a few tennis balls in the dryer will help fluff feather pillows. Polyester fillings may need to be pulled apart to fluff after drying.
Some manufacturers recommend high temperatures for drying polyester pillows. Check the label.
Foam pillows can be washed by hand if cleaning is needed. Remove the cover and wash them separately. Place the pillow in warm, mild detergent suds and squeeze the suds through thoroughly. Rinse several times, removing the detergent residue. Blot well with a towel.
Dry flat at room temperatures, reversing positions occasionally. Drying can take several days. Don’t use a dryer because of the danger of combustion.
So, it looks like what happened to my pillow was that I washed it too long, although I used the delicate cycle. Also, the cover wasn't strong enough to stand up to the washing.