While there is little doubt that light is an accelerator, as you mention, it can cause a lot of problems with algae.
When plants are in low light they will divert much of their resources to the light gathering mechanisms.
Primarily, this means the an increase of chlorophyll and the chlorophyll building/maintenance infrastructure.
This reduces their stores of energy and so there is very little left for growth. The increase in chlorophyll can result in a darker green.
Adding more light increases the growth rate by energizing the chlorophyll which can then produce sugar at a higher rate, allowing for more growth.
This is hardly surprising, however. Sticking close to the substrate is different behavioral issue. There were probably more than a single issue going on at each point in time. Very difficult to draw a unified conclusions based on different condition. It's not a safe enough assumption that because the tank was the same tank, that the conditions were the same at all times. Conditions can vary even in the same tank at the same time.
Which is not to say that light has no impact on the path or shape of the growth, only that it may play second fiddle to other factors.
Check this out: Here was my tank with C. wendetii brown and green. This tank had at least 70 different species and I had to just stuff plants in where I could. There was no room and the plants competed for space, for flow, for light and for CO2.
As you can see, the wendetii brown muscled everything out and had lots of light, flow and CO2, yet it grows more horizontally. It just sprawls and that's just how it grows. Just to the right of the brown you can see a small green being assailed by it's neighbors. It wants to grow upright but is under duress.
It has sent runners far to the left and plantlets are popping up to the right, in the extreme corner as well as to the left of the wood.
Here is that wendetii green on the right side 5 or 6 months earlier. The lighting is low to medium due to shading of the ferns but flow and CO2 is excellent.
I've zoomed in to show that wendetii green wants to grow upright and does so even when the light is not high.
And of course, when offered the same platform of strong light, flow and CO2 to compete on even ground with the brown, free of encumbrances, the plantlet on the left of the wood grows vertically.
This is just an example of how the shape of the plant can be attributed to many combination of effects. Although the light varied, what is for certain is that flow and CO2 were as much as possible, held consistently excellent.
The wendetii brown as far as I can tell prefers to grow in a sprawling horizontal manner when the conditions are high light/flow/CO2.
I did not grow it under any combination of low light/low flow/low CO2 so I cannot conclude that it grows this way under all conditions.
Cheers,