Physique is a measure of how much your pet weighs in Nintendogs + Cats. There are five weight conditions. They are: Overfed, Plump, Ideal, Skinny, and Underfed.
On the other hand (or paw in this case), if a pet is Skinny or Underfed, they will require Wet Food. The player will also need to go to Cafe Petrov as much as possible. One must always remember that the food must be eaten in moderation.
Which brings the idea of Ideal weight. This is achieved by exercising regularly, but not too much. Also, If the player has a dog, feed it treats every so often on a walk to keep its metabolism high. As said above, moderation is key. Don't overwork pets, but don't let them be lazy either.