Your Papillon should not be over- or under weight. Both cases can be a sign of ill health, or it will lead to ill health.
If you don't know if your Papi is over- or under weight, here is what to do:
Run with your hands on both sides along his mid section (ribs). Feel the ribs? If you don't, your dog is surely over weight. Are the ribs very, very pronounced? Feel every little bit of them? Your Papi is clearly too thin.
So: you should feel the Papillons ribs, but they should not protrude too much.
Road's End Mickey