
The Pebbly stuff that the breeder fed her on will have been a complete food like the Bakers, but minus the shapes and colours, that are there purely to appeal to you the buyer. Some of these foods also contain a lot of sugar (will say corn syrup, caramel or Fructose) which will make it more appealing, but less healthy for the dog.
It is a bit late to tell you that it is best not to chamge, or only slowly changefoods, and that it is common for Pups to be off their food for a few deays in their new home.
Hopefully you will have had a diet sheet or guidance with the food the breeder gave.
Spaniels are often kept fat, and puppy should have a waist and ribs be felt easily, but not showing.