Lightening a mass of a bigger diameter reduces more inertia than doing so on a mass of a smaller diameter.
I = mr^2 (Inertial = mass x radius x radius)
Hypothetical examples:
Flywheel, 5kg --> 4kg or -25%, 300mm. Inertial reduced by 0.0225kgm^2
Pulley, 2kg --> 1kg or -50%, 200mm. Inertial...