Here are the reasons why:
Toy, and small breeds in general have much larger energy requirements (per pound of body weight), than their medium + large brothers.
The energy requirements for your Papillon, or small breed puppy is 2-4 times higher than of it's adult version.
Puppies, especially in their first 6 months, also need more minerals and protein in their diet, for their bone + teeth development.
Small dogs have small stomachs, but they need to eat large amounts of food in relation to their size/ body weight. They need small meals, given several times / day, to fulfill these requirements.
Kibbles, in bite -size, is the best dog food for our small companions.