possible competitive eating issue?

i have 3 cats, all male & not neutered yet (i have an upcoming appointment in august to do so). i noticed weight loss in 2 cats, weight gain in 1 cat, i took one cat to the vet because of his weight loss and now i noticed it in one other cat. all of them are acting themselves, no signs of pain or discomfort, no vomiting, no diarrhea, and they do eat. the vet said it’s that they need to be neutered to gain weight.