Underfoot Combat( Races of the Wild, p. 152)
You can enter the space that a foe at least two size categories bigger than you occupies.
Tumble 10 ranks, Small or smaller,
Confound the Big Folk (RW) ,
You can move into or through a square occupied by a creature at least two size categories larger than you. You do not provoke attacks of opportunity for doing so. While you are in a square occupied by a creature at least two size categories larger than you, you gain the benefit of soft cover (+4 bonus to AC) against all attacks, including those of the creature whose space you occupy.
Without this feat, you can move through squares occupied by a creature at least three size categories larger than you, or a creature three size categories smaller than you (or any creature, if you are Tiny or smaller).