Basically, it boils down to a server specific problem. There are some PHP installs where getimagesize() just doesn't work, so there is no way for it to figure out the width/height of the image so it defaults to the config size.
In 7.2, since we now have the option of using GD2 or ImageMagick for CAPTCHA, I'll see if there is a way we can use those to determine the size of the image when getimagesize() doesn't work.
