all the above elements come into play, yes by the lights being at the optimum distance from the plant the strech of the plant will be reduced and the stem would thinken.
but as we all know not everybody will use the lights properly and a oc fan or pc fan can help to thicken the stems. i oc fan is not nessery if the ventilation is adiqut in the grow room. sorry for the spelling its not great.

email you know more then me, so if you understand what am saying and am wrong the pls correct me .