The module gets the thumbnails directly from Youtube... But it seems the YouTube API has changed somewhat since this module was built. So the stylesheet still thinks it will get a small thumbnail and sets the encompassing div to a width of 150px, where actually now the large thumbnail is fetched from google.

If you want the small thumbnail, it'll will need some hacking - there's another post about it here that might give you some idea:

http://silverstripe.org/all-other-modules/show/14569

If you want to stick to the large ones, you'll have to edit the YoutubeGallery.css stylesheet (you might want to anyway )...