The material below is Open Game Content from a 3PP Source
You slip under and around your larger foes.
Prerequisites: Dodge, Mobility, Small size or smaller.
Benefit: You receive a +4 dodge bonus on Acrobatics checks to move past opponents without provoking attacks of opportunity, so long as they are larger than you. In addition, you gain a +2 dodge bonus to Armor Class against attacks of opportunity caused when you move out of or within a larger opponent's threatened area (this stacks with the +4 bonus provided by Mobility, for a total bonus of +6).
As a racial characteristic1
Underfoot: Halflings must train hard to effectively fight bigger opponents. Halflings with this racial trait gain a +1 dodge bonus to AC against foes larger than themselves and a +1 bonus on Reflex saving throws to avoid trample attacks. This racial trait replaces halfling luck.