You are correct that the larger trees have more surface area with which to complete photosynthisis; however, that does not necessarily mean that a larger tree is better than a small tree when acting as a carbon sink...

A large tree, for example may use a tremendous amount of its energy on reproduction, as opposed to tree growth. Or it may increase leaf production which would be worse than wood production from a greenhouse gas prospective. (More leaves may not be a bad thing when looking at CO2 consumption; however efficiency must be measured in the real world based on more than just CO2 consumption. for example, A key factor to consider would be methane production that results from trees sheding their leaves...Leaves that are composted by bacteria into more effiecent greenhouse gases such as methane...So the question then might become is a hardwood (oak, maple, etc...) as efficient at removing carbon as a trees that doesn't lose its leaves (such as Softwoods, Bamboos, etc...)?

Hell I don't know the answer I'm just an accountant that can tell you the greenhouse theory doesn't add up... One thing is absolutly certain: There is more to global warming than just SUV CO2 emissions.

(At least in the real world)

Scott