I'm just reaching for a limb here, but...
Dixie cups aren't lightproof. So could it not have had some root trimming affect when it came into contact with the edge of the pot and the light. B/c the light changes roots, turns them green and confuses them, which to me would stop or slow growth. So naturally the roots in the middle would have to pick up the slack. In theory roots extend to seek out more needed nutrients but if you were very carefull to aways have them provided in the proper balance the roots should spend more time growing the plant and less energy growing roots.

The only other thing I can think of is actual root trimming, I hack off the outside of the root ball on my mothers every couple of months to keep them bonzia.

Then again I'm just thinking out loud.

Oh and even though it's not your pic you still snuck some rep out of me.
Chronic Chrissy Reviewed by Chronic Chrissy on . The 1 oz. Shot Glass Grow Competition. yes, that's right. we will be growing a plant in a 1 oz. shot glass. no flamers please. this is an experiment for fun only. chances of hermie are great, chances of male (if from bean) are high. we KNOW this; the poing is to see if the plant can survive in that small of a container. weve all seen weeds growing up through cracks in the side walk, this is the same principle. rules: you must use a 1 oz shot glass the shot glass must be clear, for honor system you may veg for 5 days Rating: 5