Rashi explained that "Arich Al Gabei Arich" means that the upper written part is aligned with the lower written part, and is not over the empty part, as opposed to "Arich Al Gabei Leveinah," in which the written part is over the empty part. The reason why Ha'azinu is "Arich Al Gabei Arich" is that it also includes calamities in it -- for Resha'im ("Harninu Go'yim Amo Ki Dam Avadav Yikom... me'Rosh Paros Oyev") (Ran). The Ran offers an alternative explanation in the name of Rabeinu Tam, that the "Arich" is for a short word (as an Arich is half the size of a Leveinah; see Eruvin 13b), and Leveinah is for a long word. Thus, in the Shirah of Haman, the Es is a short word and the name of Haman's son is a long word, and that is the Arich and Leveinah. Once they are both straight, they resemble a building from which Haman's sons fell and could not find any foothold on which to hold, as opposed to the Shirah where there were "footholds" in each sentence, as if there were some way to be saved.

Ha'azinu, however, has long sentences and not just one word, and therefore does not belong to this type of Shirah. The Gemara here is referring only to those types which have Arich and Leveinah.

Yoel Domb