In a given area A, we have two choices - (i) we can plant maximum number of trees (which are larger in size) possible in A, say m OR (ii) we can plant maximum number of plants (which are smaller in size) possible in A, say n (evidently m will be smaller than n) Which choice will yield more amount of oxygen? It can be assumed that the total number of leaves capable of Photosynthesis is N and is same for both the cases.
Ignoring parameters such as:
- Leaf shapes
- Difference in photosynthetic efficiency due to other metabolic factors
- Unequal illumination of leaves
- Nutrient content of the soil
Photosynthesis rate of a plant1 depends on the [total number of leaves] × [surface area of a leaf].
Assuming that a tree occupies same ground area as a shrub, there will be same number of trees as shrubs.
A tree is taller and can bear more number of leaves and if we assume that the surface area of the leaves are similar between trees and shrubs then the tree would carry out more photosynthesis.
However you say that the number of leaves is the same; in this case both will carry out same amount of photosynthesis (the shrub may outlive the tree though, as the latter won't be able to support its huge size with same level of photosynthesis as the shrub).
A note on phtoplanktons and algae: One reason amongst a few for why they are efficient at photosynthesis is because the number of photosynthetic cells per volume is higher in them.
1 I am using the term plant to denote all plants. Big trees are called trees and small plants are called shrubs, for convenience.