Sounds like it's probably dropsy.

Are the eyes bulging also? Swelling up like a balloon and scales pine coning  are typical signs of dropsy.

Dropsy can be caused by the environment such as water and temperature (water quality, water temperature shock, pH, viral, bacterial , or parasitic infections).  Fish swelling up without pine coning probably point to kidney failure. If the kidneys are failing but the fish are still secreting ammonia, then  the condition is probably curable.

First thing i would do is check your pond  water paramaters for ammonia, nitrites, nitrates, pH.   If any of the readings are off, remove the fish and place them in a clean aquarium with good water quality. You can also add a little salt and antibacterial medication to help them along in case there is an infection also.  Feeding them medicated food probably would help too.  Did the pond experience a drastic change in water temperature recently?