According to Capcom's VP of strategic planning and development, Christian Svensson, the file size is because all assets for the DLC are being pulled from the retail disc; however, the download itself is programming code that "does not exist on the disc."
"Keys are 100K or less. It is not a key," Svensson replied to a forum thread regarding IGN's claim. We're not programmers but it sounds feasible for file size to remain slim for content that pulls all assets from the disc, but you're the consumer. Do you believe Capcom?