Kudos to SiliconDream, who pointed out an interesting forum post from Bungie's Matt Soell on halo.bungie.org's forum. A particularily interesting excerpt for you naysayers out there:

The marine was redesigned because we had aesthetic problems with it, not technical ones. This is what we mean when we say that things change over the course of the game's development. Having spent many months looking at the old marine, we revised him to make him better. He wasn't slowing the game down; he was just bugging us.

And as for the second half of the headline, well, you'll just have to go and read the post for that explanation. Ah, for the luxury of a lazy day and Halo build to explore the local wildlife... err...