well there is a method called SOG which denys this nessecity,
However root space directly correlates to final yeild...
Hence it is not possible to get a 4oz plant out of a dixie cup..
I use half gallon grow bags, with 1 plant with no veg time at all..
average is not equal to my identical 1 gallon plants with the same settings, hence if they have more root space they will yeild more..
Root bound mostly applies to plants with veg time of 1 month or more...