If the small size is related to his height alone, that’s normal for Bassets. They’re small in height but pretty long length wise, to the point where they’re almost considered a large breed of dog. However, if the issue is that he’s small in overall size, it could mean one of two things. On one hand, your Basset Hound puppy could’ve come from smaller parents, ones who are genetically predisposed to not be as big as other dogs of their kind, and that gene could’ve been inherited by your dear little doggo. If that’s the case, then there’s not much to worry about. On the other hand, it could be due to some underlying health issue, malnourishment or intestinal worms being the most likely suspects, but it could also be something else. It could’ve come from something that happened when he was a pup which caused this developmental defect. Depending on when you spot this, it may or may not be remedied. Whatever the case may be, I suggest taking him for a vet visit to get checked up and talking to your dog’s vet about it to see how serious the issue may be, and if anything can be done about it in the first place.