Actually, this is probably a greater problem with the template we need to address. Not every example will have a sun blade, after all.
The way I see it, we have four options:
1) A dire lycanthrope's weapons supernaturally resize with it.
2) A dire lycanthrope's weapons remain unchanged, and the 'thrope suffers no penalty for fighting with weapons sized for smaller creatures.
3) Give 'em all the oversized weapon ability (like the titan), and have the hybrid form be the norm, and the humanoid form fight with bigger weapons.
4) Write up some complicated mess of an ability to detail how to treat its humanoid-sized weapons as equivalent smaller weapon while in hybrid form.
Obviously, #4 is my least favorite. I'm probably more enamored with #2 or #3 than #1.