I think arguing about the rigid word limits is relatively fruitless. Realistically, as long as your aggregate word count is within the reasonable bounds for a timely essay read-- I think it's safe to assume that the adcom will still take interest in what you have to say, regardless of how your words are allocated to each individual essay. I can't imagine a GSB adcom reading somebody's Essay A and think to herself, "This candidate only wrote 550 words on what matters most to him. Clearly, the world matters less to him than that other candidate who wrote 800 words."
I think the conclusion you can draw, however, is that Stanford is particularly focused onwhat makes you tick, what's important to you, and how you have manifested those interests/goals in all aspects of your life. I think it's equally safe to say that colorful anecdotes of "what matters most to you" can just as easily appear in other Essays (B, C), even if it's not in Essay A. And while you won't be fighting world hunger in every single essay, I think overarching themes of "empathy" and "thinking outside yourself" (or whatever matters most to you) can easily transcend multiple essay topics.
So in my opinion, the main takeaway is to focus more on what you're writing and how you're writing it, and less so where you're writing it.