I just noticed that your advancement is odd.

Advancement: 4-8 (medium); 9-20(Large);21-30(Huge)
First, you should only show how it can be advanced using more hit dice. You don't need to show that it can be reduced. Also, advancing creature's usually follows that they advance in size after doubling their hit dice. Here's how a standard progression woud look:

Advancement: 7-8 (Medium); 9-16 (Large) 17-24 (Huge)

If you wanted, you could advance to Gargantuan (25-32), but generally speaking advancement only follows two increments. I know advancement doesn't HAVE to fit this model but this is fairly standard.

I believe the guidelines came from a WotC article but since I don't have my gaming stuff handy, I'm not 100% sure.

Debby