Hes not fat by any means but the vet is probably after perfect condition, last rib vissible and more of an abdomen tuck. If you want to follow the vets advice just reduce food by half a cup a day, and reasses in a few weeks. Feed by body condition is generally the best rule, feeding schedules on the back of dog food are just a general guide and are not suitable for every dog, take away or add more food based on body condition and you wont go wrong. The plus side of feeding less will be increase food drive and more enjoyable obedience training, and a healthier dog.