I agree -- not a stupid question! Let me make a guess. Photosynthesis has a capacity
for energy production related to perhaps 1/3 to 1/2 of the surface area of the
organism. Energy consumption on the other hand is more likely related to the volume.
So we have an X^2 vs X^3 problem. As I get larger or require more energy, photosyntesis
as my sole DIRECT means of fixing energy becomes limiting and I'm better off using
a food chain -- that is eating something that ate something .... that used the sun to
fix energy. So I'd venture the quess that at some point -- like when you want to be
mobile -- the energy budget says being green isn't enough. You might also want to consider
the advantages of staying out of the light to avoid being eaten.