Well, Can you think of any use for which a hatchet would be better? The only advantage I see is saving a few ounces.
Weight isn't the only concern associated with packability. I can strap a 14" hatchet to my belt and while it's not light enough that I simply forget its there, I can still comfortably sit, kneel and use a full range of motion without the hatchet snagging or bumping into everything. This is definitely not the case with an 18+" small axe - that size automatically means it goes on or in the pack, where a boy's axe could go. Speaking of which, you have to then ask yourself what an 18" axe can do that a 28" boy's axe can't? The only thing would be finer detail work, and that's what a hatchet would excel at.
If you're looking at a two axe setup, there's a lot of unnecessary crossover in tasks with an 18" small forest and 28" boy's axe. On the other hand, a two axe setup composed of a small hatchet and 28" boy's axe diversifies and extends the range of possible tasks - the small hatchet takes care of kindling, carving, bushcraft, and other fine detail work while the boy's axe takes care of heavier axe chores like felling, bucking, limbing, splitting and other wood processing.
Honestly I just don't see the appeal of a 18" axe in any situation, whether it be a single or two axe loadout scenario. The length is just too awkward to properly make a full two-handed swing and it doesn't really excel at the finer tasks either. Even Ray Mears -the man who made the Small Forest Axe famous- decided on a 24" haft for his collaborative effort with Gransfors Bruks (the Ray Mears Wilderness axe) and I'd say that's about the bare minimum needed for a two-handed axe.
If I was forced to only take one axe along, it would probably be a 26-28" boy's axe with a 2 to 2.5lb head. That size by far accomplishes the widest range of tasks, from felling trees to detailed carving (so long as you have the forearm strength for it). If we're talking a two axe loadout though, it's hard to beat a hatchet for the belt and a boy's axe for the pack (in my case substituting the boy's axe for a 3lb on a 31" handle as I've used axes long enough to find that a comfortable do-it-all).