It's the change in exposure that counts, and it is logarithmic.
Originally Posted by DannL
Take an example. You have a negative that you like, and a pleasing 8x10 print results from an 8 second exposure.
You decide to print it big - 16x20. Everything being equal, a 32 second exposure at the same aperture will yield similar tones in the bigger print.
Now, you decide you want to try it darker, for a more moody effect. You experiment a bit, and decide that the 8x10 dark version looks best with one stop more exposure - 16 seconds (an increase of 8 seconds). If you want the same result on the big print, you don't increase that exposure by 8 seconds - instead you increase that exposure by 32 seconds, to get the same one stop increase in density.
Now the numbers here are simplified, and there are other real-life issues to contend with (reciprocity, shift in contrast, etc.) but basically this works.