They are going to lower all the numbers in the next expansion. They
have to. It's gotten to the point where heroic 25 man bosses are pushing the literal upper limits to what the code can handle in terms of HP.
Assuming they are using a 32 bit signed integer (which I'm pretty sure they are given some comments they've made), the highest HP a boss can theoretically have is 2,147,483,647. 2.14 billion. One of the fights in 25-man heroic in the previous tier had 1,962,616,448 health. There were some mechanics for boosted player damage there, but it's getting dangerously close to the cap.
In the Garrosh fight even, he doesn't have one health bar with multiple phases at various percents. Instead his health gets low, then he heals to full and the next phase starts. I like to think this is because they simply couldn't fit a single healthbar in, the code wouldn't allow numbers that high.
So what happens when health
does exceed those numbers? Things
like this. There will be an "item squish" next expansion reducing the numbers. They simply have to.
Edited, Sep 27th 2013 6:54pm by IDrownFish