This is my exact same issue with Cooper. The xs is fairly small but the small is huge. I kept the xs and even though it looks tiny on him, it is functional.
Here is Cooper:
http://www.yorkietalk.com/forums/att...undershirt.jpg
Here is Max:
http://www.yorkietalk.com/forums/att...undershirt.jpg
They look like 1/2 shirts. They need to be snug around the chest and I just wasn't getting that from the size small.