Usually if a food is marketed as for small dogs it just means the size of the kibble is smaller. Most of the brands that make these formulas though make pretty crappy food to begin with and I wouldn't buy any of their claims of nutritional differences between their formulas being of added value to your dog.
Yes, some breeds do have slightly different nutritional sensitivities and quirks, but you're far better off just treating your dog like an individual dog and finding a food that works best for her.
As a rule, avoid any foods that are sold in grocery stores and at the vet's office (unfortunately vets sell some incredibly bad foods and lead people to believe otherwise). If you have a Global Pet Foods store near you, I find they carry some of the best brands out there including frozen raw and dehydrated raw which will always be far better then any kibble for your dog and with a little dog will not cost a fortune.