Here is a photo of Jackson in a sweater with it on the 2nd to last hole
http://i1085.photobucket.com/albums/...1/IMG_3580.png
Technically, Jackson's girth is nearly 19" (I think it's like 18.7" when measured out, I just checked my phone where I saved his measurements). so there's no way it's only to 18" at the top girth size, it wouldn't fit him on 3 different holes.