I found this bit of information under Kaiju on the PRD:
Massive (Ex) Because kaiju are so massive, uneven ground and other terrain features that form difficult terrain generally pose no significant hindrance to a kaiju's movement, though areas of forest or settlements are considered difficult terrain to a kaiju. A Huge or smaller creature can move through any square occupied by a kaiju, or vice-versa. A kaiju can make attacks of opportunity only against foes that are Huge or larger, and can be flanked only by Huge or larger foes. A kaiju gains a bonus for being on higher ground only if its entire space is on higher ground than that of its target. It's possible for a Huge or smaller creature to climb a kaiju—this generally requires a successful DC 30 check, and unlike the normal rules about kaiju and attacks of opportunity, a Small or larger creature that climbs on a kaiju's body provokes an attack of opportunity from the monster.
