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Weapon Size: Every weapon has a size category. This designation indicates the size of the creature for which the weapon was designed.

A weapon's size category isn't the same as its size as an object. Instead, a weapon's size category is keyed to the size of the intended wielder. In general, a light weapon is an object two size categories smaller than the wielder, a one-handed weapon is an object one size category smaller than the wielder, and a two-handed weapon is an object of the same size category as the wielder.

The implications here are that a weapons classification of light, one-handed, and two-handed is directly based on the size of the weapon compared to the user. This is why a a one-handed large weapon is considered to be a two-handed medium weapon, because of the large weapons size compared to the medium user (it's as big as they are).

Inappropriately Sized Weapons: A creature can't make optimum use of a weapon that isn't properly sized for it. A cumulative –2 penalty applies on attack rolls for each size category of difference between the size of its intended wielder and the size of its actual wielder. If the creature isn't proficient with the weapon, a –4 nonproficiency penalty also applies..

The implication here is that a weapon designed to be used as a one-handed large sword does not fall into the appropriately sized category for a medium wielder, even if it can be used with two hands. Therefore, even with Jotungrip, a Titan Mauler (or any medium creature) may not use a one-handed large or light huge weapon with one hand.

The measure of how much effort it takes to use a weapon (whether the weapon is designated as a light, one-handed, or two-handed weapon for a particular wielder) is altered by one step for each size category of difference between the wielder's size and the size of the creature for which the weapon was designed. For example, a Small creature would wield a Medium one-handed weapon as a two-handed weapon. If a weapon's designation would be changed to something other than light, one-handed, or two-handed by this alteration, the creature can't wield the weapon at all.

The implication here is that a weapon, designed for a larger or smaller sized creature than the wielder, is only usable if it is no less than two size categories smaller (light) or up to the same size (two-handed) as the creature wielding it. So, because of their "inappropriate size" they are designed for, Jotungrip disallows the use of large one-handed weapons as two-handed weapons that can be wielded with one hand. A Titan Mauler (or any medium creature for that matter) may still wield a large one-handed weapon or a huge light weapon as a two-handed weapon. The penalties are cumulative for each size category away from "appropriately sized" that the weapon gets. However, the Titan Mauler has the advantage of Massive Weapons (EX), which allows them the reduce and eventually negate the penalties to using "inappropriately sized" weapons.

Can a medium Titan Mauler (or any medium creature) wield a large one-handed weapon, or a huge light weapon, as a medium two-handed weapon in a single hand with Jotungrip?
No.

Can a medium Titan Mauler wield a large one-handed weapon, or a huge light weapon, as a medium two-handed weapon with two hands, but get reduced penalties with Massive Weapon (EX)?
Yes.

These are the most definitive explanations I could surmise.