I have a 9 lb chihuahua mix with a chest measurement right around 14 in, but he's very slender. I was super excited for the XXS size to come out because I love these harnesses, but I wish it was a bit more substantial. In the photos, you can see the size differences between the XXS skull harness and an XS jellybean harness. The plastic buckles on the XXS are quite tiny and a little harder to unclip, so people with mobility issues in their hands may struggle a bit. My dog fits well in the XXS but there's no room for a sweater underneath. He's a bit too skinny for the XS even with adjustments, but it does have room for a sweater. If he weighed substantially more or pulled hard on the leash, I'm not sure I'd want to keep him in the XXS.