First, one tiny thing to clear up. A Titan Mauler (or anyone, for that matter) can wield a one-handed or light weapon one size bigger or a light weapon two sizes bigger. So you can wield a Huge Dagger just fine, albeit with a -4 penalty by default; you aren't just limited to Large 1-h/light weapons.
But the crux of the issue is that wielding a Large Longsword for 2d6 damage is nice, but you could wield a Medium Greatsword to the exact same effect, but without the -2 built-in penalty and Jotungrip allows you to one-hand the Greatsword if you need to (ie. you're grappled or have one hand occupied) while you don't have that option for the Large Longsword. To do better than this, you'd need to use an Exotic weapon like the Bastard Sword which would only go up to 2d8. The only Martial weapon that can match this is the Scizore and it has its own issues. And 2d8 compared to 2d6 is only 2 average points of damage increase and that will get drowned out by static bonuses in good time anyway. Even a Huge Shortsword would deal the same 2d6 damage as a Medium Greatsword. Only a Huge Aklys would do better, requiring EWP for 3d6 damage. Well, at least insofar as non-magical options. You could wield a Huge Sunblade as a Huge Shortsword (light weapon) for 3d8.
One set of changes I'd suggest is as follows:
1) Jotungrip simply lowers the effort category of a weapon sized for you by one step. This means that a 2-h can be treated as 1-h and a 1-h can be treated as light.
2) While Raging, Jotungrip can be applied to larger weapons.
Regarding reach, keep in mind that a Longspear or other reach weapon is balanced to use as such while a Sword, even a large or huge one, isn't. So it isn't going to be "reach" so much as a "lunge" so I'd say that a Titan Mauler using a weapon at least two categories larger than herself, or a Reach weapon at least one category larger, can benefit from the Lunge feat without spending a feat slot on it. So, what you end up with is as follows:
You can wield a 2-h as if it were 1-h, or you can wield a 1-h as if it were light, for -2 attack penalty. While Raging, you can apply this benefit to larger weapons which allows you to wield (as a Medium character), up to a Large 2-h, a Huge 1-h, or a Gargantuan Light weapon. If using a weapon that would, without this ability, be too big for you to wield, or a reach weapon that would be two-handed without this ability, you can use the Lunge feat as if you had it. You must use the weapon with Jotungrip to benefit from this; ie. you must wield your Longspear one-handed to have enough leeway to Lunge with it. This means you can Lunge without taking the Lunge feat with a Medium 2-h Reach weapon (ie. Longspear), a Large 2-h weapon (ie. Large Greatsword) Large 1-h reach weapon (ie. Large Whip), a Huge 1-h weapon (ie. Huge Longsword), or a Gargantuan Light weapon (ie. Garg Shortsword). In the case of weapons that spoof handling category (Sunblade), use the base weapon type to reflect the actual size of the weapon.
Massive Weapons: If you have reduced the inappropriate sized weapon penalty to 0 for a given weapon with this ability, the next time you would get a penalty reduction, you may, instead, reduce the penalty of Jotungrip to -1.
This does two things; 1) it marries the Jotungrip and Massive Weapons abilities to make them more interactive and 2) it keeps the flavor of the class without going completely crazy with options to wield Gargantuan or Colossal Greatswords. And the Lunge benefit is merely thematically fitting icing on the cake.
