Actually, why do the cherub and seraph grow a size category with their first HD advancement? The planetar, solar, and astra deva all start with X HD and advance as X+1 - 1.5 X HD (same), 1.5 X + 1 - 3X HD (one larger). That's what I was just doing with the throne. The cherub and seraph have X+1 - 1.5X HD (one larger), 1.5X+1 - 3X HD (two larger). Is there a reason we did that?