The photosynthetic pathway does hold the answer to your question, BK.
The chemical reaction of photosynthesis occurs in two stages. The daytime reaction cleaves water into oxygen, free protons (hydrogen ions), electrons. Oxygen is 'pearled' as a by-product. The dark reaction, for which sunlight is not required, utilizes the protons and electrons generated during the daytime to convert carbon dioxide into carbohydrates. These carbohydrates translate into the observable growth of the plant specimens. By prolonging the period for the dark reaction, the plants have the opportunity to focus on increased tissue construction; it looks you've discovered the optimum balance of your daytime vs. nightime reaction cycles. Incidentally, there is some evidence that fertilization might be more effective when provided proximal to the start of the nightime reaction, when plants are more likely to perform uptake of nutrients from the water column to support active growth.
One of the reasons I want my kids to participate in aquarium keeping is because it provides a fantastic way to apply practical chemistry and biology!
