If the premise is increase from the initial piston, then the reference area is 12.57 in sq.
The increase in area is 7.07 in sq.
So when phrased that way, 5" piston has 7.07" square inches more than the 4" piston.
Same as saying 56 % increase in area.
How much is a 56% increase in the area of a 4" piston.
0.56 x 12.57 in sq. = 7.04 in sq
Close enough considering I was using 22/7 for PI.
So it the percentage depends on what the comparison.
If I bought eggs for a 1.00 last week and this week I said the price went up 50%, how much did eggs cost this week?