Quote Originally Posted by Board of Command
Lots of big names on that voice cast.
Not just the voice cast. There are Type-Moon people behind the original story and art. Although I don't know if everybody consider them big, but for me they are big.

There was a lot going on in this first episode justifying a bigger file size, but it's still a good question whether they really needed to go well above 400. All in all it seems to be a growing trend these days. Although if you intended to burn all 13 eps on a single DVDR, it would already mean a lot smaller episodes, so maybe they thought that if it's going to be 2 anyway, no need to hold back.