The main hormone responsible for photosynthesis is cytokinin. This is what processes the light and turns it into sugars for use by the plant in growth and maturity.
Below a certain low amount of light, cannabis will not grow as big nor mature as fast in veg'. as the cytokinin hormone (which responds to light) is less active.
A study was done, I haven't got the link, where a seed was germinated in the dark and allowed to veg for a short time after being heavily dosed with cytokinin. It grew in complete darkness, and it was noted that it's cotelydons were of a much shorter, or more abrupt stature than normal when in the presence of light. So this suggests to me that light also contributes in the build-up of this hormone, and that the more of it there is, the more of this hormone will be available. The experiment went no further, maybe they ran out of hormones, i dunno.
I hope I have that right, maybe someone could check... don't take my word for it please. We need to learn together, and it is easy to get tiny points mixed up. Science is still learning too.