Welcome to the board RGW!
I'm really not at all sure but could this be related a little bit to being overweight? Would you say your cat was overweight at any point either before the tumour surgery or after?
I ask because 12 to 9 pounds is a significant weight loss...but it may be less significant if the cat was overweight. Would you say your cat has a large, medium or small frame?
Is your vet also suggesting that the cat is now underweight? Is your vet concerned about the cat's weight, or is this mostly your concern?