Kodak Single-Use kit's instruction says to use 260mL for two 135-36 rolls, if I remember correctly. So I would suggest a temperature-related problem (or exposure!) instead of too low chemical activity due to low volume. It's true that 120 roll has a bit higher surface area than a 135-36 roll, but I believe that KODAK's figure has also some safety zone in it. This is because they can't know about the exposure, which has a large influence in the chemical exhaustion. I would guess that the effects of too low solution volume start appearing visually if you have a roll of very bright, overexposed shots.
Originally Posted by jrydberg