When the current generation began, downloadable games were small, usually retro affairs. We've since moved into downloads being the norm even for bigger retail releases, so the distinction has gotten a bit dated. With the next generation looming, it appears Microsoft will finally eliminate the last vestiges of that separation by making achievements all even-steven.
Xbox 360 Achievements lists 1000 gamerscore for Halo Spartan Assault on the Xbox One, as compared to 400 on the Xbox 360 version of the game. Peggle 2 will also sport 1000g, as confirmed by the producer. Those two would seem to signal that downloadable games that would ordinarily be restricted to the 400 point cap will be on-par with full retail releases on the next generation.
This move would align with previous statements from Microsoft about removing the designation of certain games as Arcade and others as Games on Demand. Game Studios general manager Matt Booty told us that in the new system, "games are games" regardless of indie or AAA status. Removing the achievement gap is another move in that direction.