Microsoft explains to GameSpot that the 50MB cap is so that XBLA titles can fit on memory cards and played on a friend's console, and that SotN would be an exception to the rule. Though we do enjoy the fast downloads, now that background downloading is possible (and hard drives potentially expanding), perhaps Microsoft ought to raise that size limit to a point where all of Lumines Live can be downloaded as one purchase.
Sony has implemented a cap of 500MB for downloadable titles, though the downloads currently (and annoyingly) take center stage. Symphony of the Night, when it is released (date as of yet unknown), will have a demo that does stay under the 50MB limit.
Enough talk, have at this Castlevania XBLA video