in some cases the role of leaves for photosynthesis are overestimated: BRASSICA species for instance: remove all the leaves of a plant and the yield will merely change: because the main locus for photosynthetic production are the pod walls!

Reducing leaf size will directly affect the photosynthetic efficiency of your plants.  

In my opinion, When the current leaves size are reduced, plant could undergo one of these processess:

1) plants would invest in growing more leaves. so you would find new emerging leaves to increase light absorption.

2) plants would invest in making more chlorophylls in the current leaves. with the same reason as above.

3) plants would reposition the current leaves so that it is perpendicular to light and thus would increase light interception.

all of these are done to make sure the plants receive enough sunlight for photosynthesis and growth.

bigger leaf may have the potential to increase photosynthetic capacity under thin population density but in high density bigger canopy photosynthesis decreased because of low light penetration to the lower canopy leaves and ultimately reduced phosphates production.

You should have mentioned your objective behind reducing leaf size. Surely there will be reduction in photosythesis but as suggested by Kenneth M Towe it can be easily compensated by increasing number of leaves.