High SIF in plane stress

Hi all!!!

I have a very basic question in Fracture Mechanics. The question is as following.

"Stree Intensity Factor (SIF) is more in plane stress problems (plasic zone size is big) than in plane strain problems (plasic zone size is small). How do we explain this, without refering or invoking energy conecpt?"

I request to give some thoughtful explanation.

With regards,

- Ramdas