my fix as a 12 year DM:
Drop the Titanic Rage ability and make a new ability that would be like the Massive Weapons ability. Make it level progressive pertaining to the wielders size increasing for the purposes of wielding inappropriately sized weapons.
For Example:
Titanic Weapons (Ex):
At 8th level and every four levels beyond 8th the wielder is considered one size category larger for the purposes of wielding oversized weapons.
8th level - Large
12th level - Huge
16th level - Gargantuan
20th level - Colossal
This ability replaces your rage power at the level in which you gain a new size category.
But with new ability comes new problems, the Jutengrip ability becomes more powerful, because with another size category for the wielder means you can wield a larger sized two-handed weapon in one hand. So just add the word "Medium" into the Jutengrip ability, and take out "The weapon must be appropriately sized for her.", like so.
Jotungrip (Ex)
At 2nd level, a titan mauler may choose to wield a Medium two-handed melee weapon in one hand with a –2 penalty on attack rolls while doing so. and it is treated as one-handed when determining the effect of Power Attack, Strength bonus to damage, and the like.
This ability replaces uncanny dodge.
This fix would allow wielding of greater sized weapons while keeping the core rules for inappropriately sized weapons preserved and still keeping the penalty for wielding such weapons. If you wanted to instill harsher penalty's still, you could add While Raging to the requirements of using such an ability.
As for it not being feasible to wield such a weapon, I would like to remind others that this is a fantasy game and it is based on level progression further into the fantastic. That's why I would allow it as long as it was a progressive ability. Besides I think with all the magic in a fantasy setting, you should be able to find a way to wield such a enormous weapon. lighter materials, magical weapon abilities, but most of all pure epic class ability should be enough to satisfy anyone and their frigid realistic outtake on a fantasy RPG game.
Hope this helps.