Traditionally, photographic enlargement is expressed in terms of magnification; that is, the linear dimensions of the print. So an 8x10 is twice as big as a 4x5. When you blow your 4x5 neg up to a 16x20 print, it's a 4x enlargement, not a 16x enlargement. And the formulas to figure out the increase in exposure when you rack the enlarger head up the column to increase the print size are based on magnification. When it comes to film size, however, many photographers compare formats by the increase in imaging area one would get by going up a size, so 8x10 is 4x as big as 4x5. But when it comes to talking about prints, an 8x10 negative is twice as "enlargeable" as 4x5 negative from the same film stock. So really both ways of thinking about sensor size are "correct". Another variable that can influence enlarge-ability with digital cameras, of course, is pixel density, a whole other can of worms - maybe the rough equivalent to a high resolving film vs a low resolving one. I think it's fine to use either definition, as long as one realizes it's not so cut and dried, and specify which one when trying to communicate with one's forum buddies . If you go to a store to buy a cardboard box and you ask for one that's twice as big as a 10"x10"x10", would you be more likely to get a 20"x20"x20" or a 12.6"x12.6"x12.6"? Hmm...