There can definetly be a size difference and even more so appearance differences. My Mom and I have sisters from the same litter. Mine is just a little bigger, but they don't look like they are even from the same State let alone the same litter. It would be helpful if we saw a photo. There really is a good chance your puppy is a purebred Yorkie. Also, they slim down after they loose their "baby fat". Maybe to help calm your nerves, you should pick your puppy up and let him show you how much he loves you
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