Except the last time when 20MB was "large" was in the early 1990s. Today, even if someone goes to clean out their harddrive, a 20MB file is unlikely to even appear on the radar.
Also, what if the mail component were used to hide/archive the virus? Hide a virus attachment from someone to themself, then have some bootstrap code (Outlook/email client exploit, perhaps) that loads the email archived virus back onto the comp.