[izacus]

Do you have any source that they're actually recompiling everything?

For example running original Xbox games on Xbox 360 also required a download of a binary which was an emulation / compatibility wrapper around the binary on the DVD that shimmed out and hotpatched code so the games worked properly.

[nitrogen]

The download sizes reported by other commenters seem to suggest a bit more than simple shims.

[chc]

The download sizes do suggest you're downloading a whole game, or at least most of one. This does not, however, imply that the game is ported -- just that they pull the game from the server rather than the disc. This makes sense even if they're emulating, because most Xbox 360 games will have received a lot of updates compared to what's on the disc, and Microsoft have probably only tested the emulation against one specific version.

[jayrox]

its also certainly possible that part of the emulation requirements include xbox 360 patch levels, an emulated file system for updates and everything else to be together in a single container.