Well, the math is a bit off, you need the ratio of the two volumes, not just the diameter.
But you are right, a larger piston pushing a smaller slave will result in more movement at the slave, the opposite will result in less movement, but with a mechanical advantage.
But that doesn't mean the sizes the Triumph engineers selected are wrong, and of course other parts of the linkage (clutch petal, the arm that operates the fork) also affect the system.