yeah the file size is really weird
its aggrivating to know that in the other thread Sega fixed it not too long ago and somehow the fixed version was taken down.
problem is in the game's coding somewhere conflicting with the DLC stuff. Some people have no problem with the DLC achievements, for others (like me) if you can't get 1 you won't get any no matter what is done even if you delete everything and restart. The 12 core achievements are no problem tho.
If you feel like taking a gamble, you could try blowing thorugh the mission mode as soon as you get access to it to see if the dlc screwed up on you too.