Because alpha = tau / I is inverse in inertia at fixed torque, increasing the moment of inertia reduces the angular acceleration. The trend is exact only for the stated torque and moment of inertia in the ideal rigid-body model. Real systems add axis choice, distributed mass, friction, flexing, bearing losses, calibration, and measurement/source uncertainty.
With the same torque, larger inertia changes spin more slowly. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Larger inertia gives a smaller alpha
The same 12 newton meter torque acting on inertia 12 gives alpha 1 per second squared.
α=12/12=1persecondsquared
The torque did not change
Only inertia changed: alpha goes from 2 down to 1.