If you are interested in working on tricks and behaviors with your dog, your dog should be the proper weight to prevent injury, discomfort and disinterest in working. A dog that is even slightly overweight can loose interest in working for food (not all dogs mind you!). If your dog is slightly overweight, try to maintain your dogs appropriate weight and see what a difference it can make in training. All dogs should have a waist!!! Depriving a dog of food is not the idea, the dog should feel comfortable not hungry. If your dog is ravenous, feed a small meal before training, to stop frustration.
I got this weight chart off the internet, I could not find who is the actual artist who created it, as it was on many internet sites without credit. So if you know where this diagram originated, please let me know so I can give them credit for their work!
A lot of vets will not tell you that your dog is overweight! It is up to you to assess your dogs weight and decrease or increase calorie intake accordingly. It is especially important with toy dogs as they can gain and loose weight rapidly if you feed a varied diet.