In Paizo's latest supplement release, Ultimate Wilderness, we see an Archetype for a class also announced in that book; the Rageshaper variant of the Shifter.
The Rageshaper has the following class feature:
Devastating Form (Su):
A rageshaper can enter a state of intense fury, transforming into a living engine of destruction. This ability functions as the barbarian’s rage class feature, except entering this state of fury requires a full-round action that provokes attacks of opportunity and has additional effects as noted below.
At the start of the rageshaper’s next turn, he not only gains the benefit of rage but also grows one size category larger (though worn equipment and held items do not increase in size). Armor or clothing the rageshaper wears that doesn’t accommodate his changing size immediately gains the broken condition.
[... text about the rest of the ability, not concerning size increases...]
At 10th level, the rageshaper can grow up to a maximum of two size categories larger with devastating form and his rage is now treated as greater rage.
At 20th level, the rageshaper can grow up to a maximum of three size categories larger with devastating form and his rage is treated as mighty rage.
My question is: Would this size increase, as written, stack with that from effects such as Enlarge Person, or Expansion?