Angelo, could you please provide us more details of this? Water parameters (ammonia, nitrite, nitrate, pH), tank size, tankmates, filtration system, tank maintenance regimen, foods being provided (aside from Oranda Gold that is), history of health issues and then photos are really needed. Photos are extremely important especially "before" and "after" shots. This is one way for us to point out what caused the wen to shrink. Without further details, then we may be unable to help rectify any possible issues that may have cause it. Does your fish have any unusual symptoms aside from shrinked wen?
A shrinking wen may indicate inadequate nutrition if all you have been feeding him is just Oranda Gold although this seems oddly the opposite since Oranda Gold is supposed to encourage the wen growth and not vice versa but there could be something to that we may find out later. It could be a sign the fish is failing its health which is why I asked for the unusual symptoms that you may have observed aside from that.