Strange. I use meteor slides on multiple domains and a range of themes to some vanilla themes to some complex themes, and have never run into this problem. But I know we can figure it out.
Did you have the dimensions in the slideshow settings set to 300x225 before you uploaded these images? The slide images are cropped to fit those settings when they are uploaded, changing the setting later updates the size of the slideshow, but not the images themselves.
With that said, you might want to do this: delete your slides from your media library ... just go to slides > slides and delete all of 'em. Then ensure the settings in your meteor slides are configured how you want 'em, and reupload. This should surely take care of your problem.