Where is your Center of Gravity (CG) located? It appears that the CG is relatively centered. You may want to bias the CG to one end of the robot.
Yes, running a larger sprocket reduces the load on the chain; however teams are running down to 16 tooth sprockets for 25 series chain (on 4" wheels I believe). Even running 6" wheels, the sprockets don't need to be nearly the size you have currently selected.
If weight becomes a concern, you can switch out those heavy metal pneumatic cylinders for some
lighter plastic ones.
Keep up the good work.