If one of your Ogres doesn't live up to it's billing, it's not as bad as when a Treant doesn't live up to it's billing. And that is a problem because if you are going to give some sort of boost to large units, where do you draw the line? An accross-the-board fix for all large units that can make Trolls worth their cost could make a unit like Ogres or Abominations (which are already quite fine where they are) too powerful for their cost.
That's why I like the Monster designation, because you're basically positing that the unit is fierce enough to use its size as a weapon itself. Because yeah, giving all Large guys an impact hit is probably the wrong way to go, but specific guys would get it. I think Ogres are right on that borderline of being a good buy vs that "not-bad but not-good" designation I put on most units. I'll post up the complete rule in this forum when I get a chance.