I really don't think they "dumb down" games for the 360. The only games that don't fit on the DVD 9 discs that 360's use are large RPG's and maybe some of the PS3 exclusives that purposely take advantage of all the extra space. The game could still probably be fit onto 360 discs without problem, so there would be no need to "dumb down" the game to shrink the file size. The hardware is basically the same, with the 360 actually having more RAM than the PS3 (not sure which RAM, but 360 has 512MB as opposed to the PS3's 256MB).
So other than the disc format/size, there would be no reason to "dumb down" the game. Multiple disc games, while they do take a whole 15 seconds out of your life when switching discs, are perfectly suitable in the case that the DVD 9 does not have enough space. And it's been done for years with Final Fantasy and other games on PS1/PS2, not just the Xbox and Xbox 360. Also, why is it that many multi-platform PS3 and 360 games look better, play better, or have more features on the 360 version? Obviously the hardware permits it, and the disc is large enough for most normal games or this would not be the case.