Here you go, Piston area and force.

Mattax said
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?

You’re right, I did my math wrong. What I was actually meaning to show (and goofed up) was percentage difference:

What I showed was percentage decrease, which is the wrong direction as the thread started with small to big:

Your percentage increase, which as you noted, matters how it is phrased, is also correct:

Accordingly, the way you phrased the egg price increase would generate $1.50 as the new price.

However, if you said that eggs cost $1.50 this week and $1.00 last week, the percentage difference would be 40%.