Paladins mostly use regular-sized shields, with a few smaller options. If you're looking for a tower shield, then yeah, you're SOL on that. (Alternately, you can glamour The Emperor's New Shield to hide it entirely.)

Warriors can get a couple of hammers. Warrior and dark knight also have a scythe available on the Mog Station.

Gunbreaker weapons are slimmer because they're one-handed weapons with a more fast-paced attack style. It'd just look silly if they had huge bulky gear like warriors and dark knights. A few of them are bigger, though; I like the Deepshadow Manatrigger, for example.