..is when they want to get more detail into a 
shot than could be accomplished with a standard 
size cel, so instead they make the cel bigger 
and hold the camera farther away.  I -believe- 
An example of that would be the Mahou Tsukai Tai 
cel I used to have that is in Ms. Poe's gallery 
now of the group in bathing suits.  There is a 
fly that buzzes around Takeo's head in that 
shot, but the only way to make the fly's wings 
visible instead of it just being a little speck 
was to make the cel bigger.  (I could be 
wrong..this may have been a pan shot but I don't 
think so)  Either way I know this is another 
reason for oversize cels.