Badger might only weigh 18 lbs

Badger is an oddly-shaped dog. He has long, spindly legs, a big square head, an underbite, and a narrow but deep chest. It's this last feature that makes it difficult to find harnesses and other wearable equipment to fit him.

Last year we purchased an Outward Hound backpack on Doggy Loot. We picked the medium size, which is said to fit dogs from 18 to 50 lbs and with a girth of 26"-32". For almost a year, the backpack hung in our closet, until this past weekend.

I loaded the backpack with one water bottle in each side pocket and tried my best to convince Badger that it was a good thing by clicking and feeding him kibble mixed with hot dogs. He wasn't entirely convinced, but he let me strap the thing to his back, and...

It was too big.

Wait, what?

Despite Badger weighing about 43 lbs, I had to tighten all the straps completely. Even so, the chest strap is a little too loose, and the strap that's supposed to go around his waist dangles uselessly without even touching him. The only strap that fit was the one that went behind his front legs.

We took him out for a walk anyway, and although the backpack stayed on, we had to adjust it about every 10 minutes because it would slip to one side or the other.

It also made sloshing noises as he walked

I'm quite confused about why the backpack doesn't fit, because Badger is squarely within the range for a size medium. I'm considering buying a small backpack to see if it fits, even though the sizing chart recommends that for dogs under 18 lbs and with a girth less than 25". Could it be that Badger has been cheating on his weigh-ins and only weighs 18 lbs?

Is your dog hard-to-fit? Have you experienced sizing issues with backpacks or any other dog gear?