I think that's intended. Like saying that if it's not strapped to your back, then it's too big to carry and effectively swing your normal weapons. So they made it so you couldn't just stuff itin your pocket, but instead have to equip it or put it in/on a backpack.
I could be wrong though. Just remember that sometimes the mechanics of a game aaren't just rules, but can imply other things. You might want to check out
Extra Credits: Mechanics as Metaphor Part 1.
Still though, that is quite a problem when you're just doing some blacksmithing. Maybe if it detected if you had a main weapon equipped, and if unequipped give the bottom 2 rows of your inventory an extra block in width. Hmm no that give problems when you try to equip a weapon though. Because then it'd have to drop the weapon, which could result in it being lost forever.
You could just unequip your normal weapon so you can carry it in the weapon slot. No... that won't work if your character normally uses one of the larger weapons. Maybe for the purposeof crafting, allow characters to pick up and carry larger items the same way they do NPCs?