I have a little chihuahua and when she runs and plays, she'll play hard but she'll run out of energy in a fairly short amount of time. Same with going on walks, after not too long she's pulling against the leash and she'd rather be carried. I kind of think of it like she's small so she has a small tank of gas to work with hehe. I don't mind it, in fact I selected this breed/size because I read they only need about 10 minutes of exercise a day, and she probably gets more than that most days. If she has a big day like going out to my parents' house, she'll sleep the whole next day almost. So anyway my point being, I think it has a lot to do with their size. I wouldn't worry about your dog too much if she seems otherwise happy and healthy. You might research your dog's breed and size and find you're worrying too much and 15 minutes or so of exercise/stimulation is all she really needs.
