I would say its just a good rule of thumb, not neccessarily set in concrete. Its just a safe guide to go by.so the general rule of thumb is one gallon of medium per foot of plant, i say horseshit. my plants are in assorted sized containers, and as an experiment, i left a few of them in their original containers to see how they would do in such small pots from germ to harvest. i must say i am well pleased. have to give em a little more tlc with water and nutes, but other than that, it seems to work fine. i wouldnt recommend this as a preferred growing style, but just showing that it is possible.