I just read your Blizz/D&D guide. Sheesh, why the heck would Blizzard be more effective on 3x3 buildings? Strange, really :p

Anyway, I've tried the following in PudDraft (with the War2Unit plugin):

Make grunts the same size as a hall (4x4) - Both spells will do exactly the same damage to them. The building flag is actually ignored by the game - All that matters is the unit's size! (Of course, in the end, that doesn't make any difference, but it's still interesting )

So Blizzard does more damage on 1x1 and 3x3 units, while D&D does more damage on 2x2 and 4x4 units. I don't think the programmers did this on purpose :p
Maybe it's kind of a bug...

Ya I was surprised when I first noticed this. I came across it simply because decay...which was supposed to be godly at killing buildings, always seemed to dick all to barracks. Then when I played humans the odd time, Blizzard would own a mound. I don't get it either...must be a mistake of some sort.