28 pounds is not undersized. I have a friend who has a young female (looking for a home by the way, anyone?) that weighs about 25 pounds. She's not super short though.
I have a very petite female right here - I think she may weigh 22 pounds soaking wet. And she is short, probably no more than 17 1/2 inches tall. She is purebred fore sure, and bred for work, not sport - she is an import from Wales and is ISDS registered.
And then there was the famous Jen. She is also bred from working lines, and was 15 1/2 inches tall. She wasn't dwarf, or stunted, she was perfectly proportioned, just incredibly petite.
Here's a pic I have of my normal sized BC Ben going over a homemade jump (these are the sorts of things you do when you have BCs - I practiced flyball in my living room!), and Jen going over the same jump - the two pics spliced together.
ETA: Your pup looks like a Mini Aussie, actually - but it could be a mix. Or, a Pom/BC mix. It's not the size, it the shape of the head and the big eyes that screams "toy dog."
Becca Shouse: Irena Farm, Semora, NC
Cord, Ted, Gus, Zhi, Maggie, Lynn, Lu, Min and Tully - and Sam!
Waiting: Bubo & Ben