There are two lines which can be uncommented to enable PP to use the smaller version, in image-inc.php, find and uncomment out the lines:
Code:
Content visible to verified customers only.
I suppose this could be a configuration item in the future, but some people use thumbnail frames than expect a specific sized thumbnail and didn't like that we did force a thumbnail to be made - it made the gallery pages look "un-uniform" (if thats a word).