V. Rogers
I think the idea is that larger dogs and animals that an owner would actually ship as cargo are many times hunting dogs, and/or animals outside a lot more than a pocket pup or cats. They may indeed harbor something just because of the nature of being outside so much, even in a pen all the time. But small under the seat animals are seldom exposed to a rabies carrying animal because they are inside more than not. Pocket pups sometime never hit the ground and can't be left to run outside because anything can kill them. That's the only logic I can think of.