Against a typical foe that's two or more size categories bigger than the attacker's own, a typical attacker's attempt to grapple stops at Step 3: Hold because the attacker "automatically lose[s] an attempt to hold if the target is two or more size categories larger than" the attacker (Player's Handbook 156). The typical attacker's bonus on grapple checks can be literally infinite, yet even infinity is insufficient to allow the attacker to take hold of a foe two or more size categories bigger than it. The best halfling grappler that ever existed can't suplex the tarrasque—or even a lowly ogre—without your help.
So, without changing the attacker's or the defender's size category or form—and without Pun-pun style shenanigans—, is there a way for an attacker to hold a foe two or more size categories bigger than the attacker?
Allow me clarify that I'm not looking for that one race that can grapple too-big creatures but for generally applicable options available to any creature after it enters play as not that race. Workarounds are unacceptable—for example, no committing suicide on a boon trap of last breath until the DM rewrites the reincarnate chart to include goliath then gaining XP sufficient to advance a level then taking the goliath barbarian 1 substitution level.
The spirit of the question would have answers locate in a text a feat, spell, magic item, or similar material that doesn't change the creature's size or form yet allows the creature to get a hold on too-big foes. A class that permits this is a last resort and shouldn't have a race as a requirement for entry.
(Be nice to answers posted prior to 5 Sept. 2017 at 1 PM PST—those answers were posted prior to the previous two paragraphs.)
Note: The Races of Faerûn feat Jotunbrud (166) is no help here: it only increases the modifiers due to size and doesn't change the creature's size category. Similarly, the Dragon #303 martial arts style Stonegrind Wrestling (57–8) says that the creature is only "one size category larger for the purposes of grapple checks," rather than grappling generally. Further, the Dragon #300 prestige class flesheater (68–9) at level 2 gains the special ability flesh rip that makes it so the creature only no longer suffers size penalties on grapple checks. Even the game's finest mundane rasslin' class, the Champions of Ruin prestige class Black Blood cultist (44–8), doesn't normally overcome this size category limit. The fan-made grappling handbook "Hulkmania Is Runnin' Wild" focuses (albeit rightly) much of its attention on becoming bigger, bypassing—not altering—this limitation it. The closest I've found is the Dragon #342 halfling barbarian 1 substitution level special ability halfling rage that lets a halfling (only) while raging (only) count as Medium (only) "whenever doing so would be beneficial, such as while in a grapple" (90–1), which is pretty darn restrictive… and, given the wording, the DM may or may not allow the special ability to work for this purpose.
It was the special ability of the prestige class flesheater that reminded me once more of this limitation: it's a halfling-only prestige class that's good at grappling… yet that doesn't really seem to take into account that some of its best abilities are unusable on creatures with a size category bigger than Medium—like bigger creatures don't have flesh to eat or something. Does anything actually change—or even eliminate—this limitation on grappling for a halfling hungry for some tasty, tasty tarrasque?
Best Answer
So, I don't read the rules the same way you are. It seems to me that establishing a hold is a grapple check, just a certain specific kind of grapple check, and so these abilities that make you larger for grapple checks make you larger for that purpose as well. In any case, I think it's clear that the authors thought it did so: that there is no benefit anywhere listed that explicitly increases one's size for the purpose of establishing a hold is telling.
Powerful Build (and the similarly-worded "Larger than Small" class feature) are clearly meant to make you count as larger for basically everything combat-maneuver-y (combat maneuvers aren't a defined thing in 3.5 but "subject to a size modifier or special size modifier for an opposed check (such as during grapple checks, bull rush attempts, and trip attempts)" is pretty clearly trying to evoke that sort of stuff, as opposed to stuff like Intimidate checks or whatever that might also work out as side benefits), and grappling is so exemplarily combat maneuver-y that it's even called out specifically in the parenthetical in most of these upgrades.
Basically, there's nothing other than straight up size-alteration that will let you do this, but I think that's because the designers thought they already had that covered (and I would argue they do already have it covered), rather than because they actually wanted small creatures to really never-ever-ever be able to initiate grapples with bigger opponents.
That said, even with this restrictive interpretation of the text, we can still grapple enormous opponents quite handily, if we can provoke them:
The step required to establish a hold only applies when one is seeking to initiate a grapple. That means the size restriction can be ignored in the prominent case of an opponent having already initiated the grapple.
The Tarrasque is built to grapple its opponents: it possesses Improved Grab, Swallow Whole, and a +81 grapple modifier. It shouldn't be terribly difficult to get it to initiate a grapple with your halfling without any effort on your part, but suggestion et. al would certainly be effective should the Tarrasque prove hesitant to bite.
Once the grapple is initiated, your halfling can pin the Tarraque, move it about, damage it, prevent it from leaving the grapple, and all that good stuff without having to worry about size limitations. The size limitations are only a problem against opponents the halfling can't taunt into initiating a grapple.