The accepted theory of one gallon per foot or one gallon for every month of life is bullshit,its been proven wrong many times in these forums,ive proven it wrong myself many times.
There is a thread on this site titled ITS ALL BULLSHIT where we discussed this,several members including myself & fdd2blk show what can be done is very small pots where people think rootbound is an issue.
When i still grew soil i grew kinda like what your talking about,i grew multiple cola plants from topping with no side branching,i grew 3 to 4 ft plants pulling between an ounce on single cola plants to around 2 ounces per multiple cola plants.
My pot size was just over 2 quarts & i made my own soil,excellent results can be had using extremely small pots,the small pot size turned out to be a blessing,you have to water most everyday & when in heavy bud 2 to 3 times a day,i ended up using a drip feed watering system for mine,it also means you feed alot more.
Using the 2 quart pots was a faster method over running large pots,the small root mass constantly feeding & drinking via drip system brought soil growing close to the speed of hydro.
The bud in my avatar was grown in a 2 quart pot,its hard to tell but that bud was as fat as a 2 litre coke bottle & longer than the bottle,members nicknamed that plant ( the moose ) & made the avatar using a pic of the bud.
You do not need big pots to pull 2 ounces,it can be done using 2 quart pots along with a vigorus watering & feeding schedule or a drip system,big pots are not needed,if your friend got tiny bids it was more likely his methods or plant genetics vs pot size.
Look up that thread its all bullshit & see routbound plants in tiny pots giving large yeilds.