Remember their bodies are very small therefore their bladders are tiny. Smaller breeds need to be let out more often than bigger breeds.
I don't think thats true, all dogs urinate the same, relative to their body size. A chihuahua will not pee the same amount as a rotti, and should be able to hold it as long as any other size dog.

The first couple of weeks are always the hardest for potty training. Just be patient with her, and she will get it sooner or later.