Is it more proportional to the mass or the volume of the plant? I thought it might be helpful to think on the cellular level here. Even a reference to an external explanation would be useful.
Welcome to Biology.SE! I am not entirely sure what your are asking but here are some places you might want to start:
Photosynthesis Molecular Biology
If you want to know about the number of sugar molecules created 'per photon' by photosynthesis you can explore the light-dependent and light-independent reactions used in photosynthesis. In particular you might want to look at the Calvin Cycle and how NADPH is used there and how NADP is reduced to NADPH in the light-dependent reactions.
Carbon Capture Efficiency
You might be interested in the variation between plants in carbon capture efficiency, i.e. not all plants are equally 'good' at photosynthesis. To think about this you might want to look at different photosynthesis mechanisms (C3 vs C4 and CAM; different forms or amounts of Rubisco enzyme) or you might want to look at sunlight 'capture' mechanisms (photosystem I and II and the accessory pigments like chlorophyll and carotenoids) or even at adaptations to different light environments (trichomes, leaf shape, etc).