Chain Plate Sizing

[Storm]

I'm in the process of replacing chain plates. Plan to copy the chain plate size currently on the boat. That brought up a question. I have a mast head sloop rig with forestay, back stay, single upper shrouds and forward and aft lower shrouds. Why would the upper shroud chain plates be the thickest (3/8 inch) and the longest (28 inches and 1-5/8 inch wide) when compared to the back stay which is 1/4 inch thick and only 20 inches long (1-5/8 inch wide). Both are using 3/8 inch diameter wire. Would not the back stay see equal if not more loading than the upper shrouds, and therefore should not the backstay chain plate be at least equal in size to the upper shroud chain plate? Also note the fwd and aft lowers use chain plates sized equal to the backstay chain plate. I should note both chain plates have survived nearly 40 years as designed, and the boat (34 footer) has seen at least 70 knots and 50 foot seas, but still want to be sure it built to standard designed specs.
Regards