Have you played a flash game lately? The initial program begins running really quickly, where it then downloads assets using a separate loader while a splash screen displays. Sony also ran into this problem with the massive files (20 - 40 GB) for their disc-based games that are also available for download, but somehow (in many cases) the games can be played while the download is happening, as the most critical parts are downloaded first.

Browser gaming may or may not take off, but I really don't see asset-size being the rope with which it hangs itself.