I think everyone is over reacting just a bit to a non-issue.
For example, consider the front caliper that I am using on my car and the size of the pistons. 1.942" od x 4.
That set up was used on the big Fords with essentially the same master cylinder bore on a 7,000 pound car since 1965.
It has no issues in stopping that monster. There is mechanical advantage working here. Why is that suddenly questioned even on a smaller caliper like the original Girling Pantera caliper?
Simply stated, there is no issue. The original braking in the car is "adequate". Many of us are just looking to improve it.
I think that Rene is just very analytical in reviewing the original design, looking for an original engineering/design flaw, which I don't think exists. He just seems to want to verify that. That is fine for his own peace of mind.
Incidentally, on pistons sizes, Ford never had an issue with using four equal size pistons in their production vehicles. The 65-7 Mustang caliper is essentially a scaled down version of the "Big Ford/T-bird" caliper and they are equal bores as well.
Varying the caliper piston sizes is just a design option some felt would reduce the likeliness of pad squeak. Since those days, other methods of eliminating pad squeak have been developed. For instance, using the pad adhesive is very effective and inexpensive.
That works. "If it ain't broken...don't fix it!"