Hoo. One of our players wants to play a barbarian with the Giant Totem and the more I dig into it, the more issues seem to arise.
To begin with, wielding weaponry of a size inappropriate to your own doesn't appear to be specifically addressed anywhere in the book -- the Items and Sizes page (191) only references cost and bulk of larger than normal weapons.
In 1E, weapons of different sizes did different amounts of damage. In 2E, it appears all weapons have exactly the same damage, whether your club is the size of a twig or the size of a tree. Can characters not normally wield weapons of an inappropriate size? Because if they can, I don't see what stops the from trading in their weapons for tiny-size equivalent (same damage, half as much bulk!)
But the giant totem is all about wielding larger weapons, so let's set that aside. Under the totem ability, it says that wielding a large weapon doubles the conditional damage bonus from raging, but imposes Sluggish 1 as long as the weapon is wielded.
Sluggish imposes a penalty equal to the increment on AC, attack rolls, DEX-based skill checks, and Reflex saves. Sluggish 1 is therefore a significant penalty, but having double the damage bonus is pretty significant as well, due to how 2E doesn't have ways of getting immense STR scores or feats like the 1E Power Attack.
Alas, you won't benefit from the conditional bonus to damage from Inspire Courage or other spells, which is kind of odd.
I feel like this works out to roughly a "push" as far as mechanics go. -2 AC is a significant penalty in 2E due to the increased likelihood to be critically hit and the tendency of monsters to have higher attack bonuses than a PC of the same level and I'm not sure the damage bonus makes up for it.
Then there are the abilities:
You grow to incredible size. You become Large and increase your reach by 5 feet until you stop raging. You have the sluggish 1 condition (see page 324) while your size is increased.
Your equipment grows with you. If you’re using the titan mauler ability, your weapon’s even larger size causes it to have the same effects as normal for that ability.
The phrase "the same effects as normal" is always problematic when establishing a concept for the first time. You have Sluggish 1 if you are wielding a larger than normal weapon. Does that mean you have Sluggish 1 twice? I'm guessing this is intended to mean that your Sluggish condition increments to Sluggish 2 in this case.
Ouch. Now you're effectively taking a -3 penalty to AC, -2 to attack rolls, -2 to Reflex, and are significantly more likely to fail a DEX skill check (which, let's face it, you were likely going to fail anyway because skill DCs tend to be no better than a coin toss for a fully optimized character). And in return you get... what?
The spell Enlarge grants a +2 conditional bonus to damage (which increases to +4 when heightened to increase the character's size to huge). Neither Giant's Stature nor Titan's Stature reference the spell nor the conditional damage bonus, so we must assume it doesn't receive any. That means the only benefit is the increased reach.
Given that attacks of opportunity are now strictly limited by class and the only way Barbarian gets anything similar is through Come And Get Me and Vengeful Strike, this would appear to make this skill extremely circumstantial, as there is now functionally very little difference between being size medium or size huge beyond forcing some opponents to take a Step or a Stride action on their turn (it mattering very little which, because you are unlikely to be able to make an attack of opportunity) or possibly managing to hit opponents who are at higher elevation.
Oh, and Giant's Lunge just increases your reach some more. Again, chances are that you won't have many ways to take advantage of this. It's not as if most monsters will connect with all three attacks anyway, so forcing them to give up an attack at -10 to get closer to you doesn't feel very strategically significant. Having an advantage over larger monsters with attacks of opportunity of their own, well... again, circumstantial situation.
All in all, it's a very flavorful feat tree that doesn't appear to have any particular mechanical benefits, working out to a push at best and usually negative on average.
Changes I would recommend:
1) Change the conditional damage bonus to a typeless damage bonus in order to allow the barbarian to benefit from allied spellcasting, and:
2) Add the same conditional damage bonuses as seen on Enlarge to the N Stature abilities.
As for Giant's Lunge, I suggest an addendum: If Giant's Lunge would enable you to make weapon or unarmed Strikes against an opponent who would ordinarily be outside your reach, that opponent is flat-footed against all such attacks.