Schneier responds to that in the comments. It comes down to "key diffusion". Not a crypto guy, but I assume this is basically a matter of statistically distribu

Schneier responds to that in the comments. It comes down to "key diffusion". Not a crypto guy, but I assume this is basically a matter of statistically distributing the key's properties as evenly and as quickly as possible. In other words, you want the key to die of "heat death" in the cipher text as soon as possible. Because the 256 bit key is longer, it's harder for it to do so. Hence more rounds are needed to further randomize the result.