Like I learned after handling it, the Volcanic is much smaller. There are subtle differences in the geometry of the receiver, hammer, and lever, but put both of them on a table together, and the Volcanic looks like a miniature Henry. Or to be more correct, the Henry since it came later, looks like an overgrown Volcanic. The Volcanic frame is just big enough to handle the 38 cal rocket ball ammo it used. When Henry developed the rimfire round for his rifle, the round was 44 cal, not 38, and in addition, it had a case, which the rocket ball ammo did not. So the action had to be slightly longer to manipulate the slightly longer round and slightly wider to handle the larger caliber. Not to different than looking at a K frame S&W, designed around the 38 Special cartridge, and an N frame S&W, designed around the 44 Special cartridge.
When I picked up the little Volcanic, it was like picking up a little 22 rimfire version of the much bigger Henry. This one was a carbine. I didn't have a tape with me, but I think the barrel looked like it was around 20" long. I think most Henrys came with a 24" barrel.
Of course it you see a lever action pistol, with a tube magazine under the barrel, that is definetly a Volcanic, as no pistol versions of the Henry were made.
Here is a quick snapshot I made of a couple of Henrys and Volcanics off of the Web. They are not all scaled the same, the Henrys aren't really that much bigger. But you might get an idea of some of the differences in the details of the two guns from this picture: