Here's the thing. You can get a German Shepherd puppy and keep it in a 4-foot cage. At some point, that dog is going to outgrow that cage. You can keep getting a bigger cage until you reach the bare minimum, but you would still have a problem because you have a highly intelligent animal is a space that just barely meets it's requirements. It's going to get bored and destructive.
Puffers may be okay in a 137, but that would be just barely surviving, not thriving and living up to it's full potential. Puffers can be trained like a dog, and they need space to swim and maneuver, just like the German Shepherd.
And yes, people keep them in smaller tanks. People keep lots of fish in too-small tanks (don't even get me started on one in a facebook group who has a hippo tang and a yellow tang in a 29gal biocube with 2 clowns and 3 damsels). It doesn't make it right or fair to the fish.