I think you are missing several factors, as Erg mode has a control loop that applies braking force if power exceeds target.
That braking force gets transmitted to crank thru gears, and counteracting the braking force requires less torque when in small chaining at same cadence.
Torque is proportional to force on pedals, so less torque requires less change in force applied to pedals.
So at the same rpm, and same power output, the small chainring requires less pedal force to adjust to changes in force that Kickr is constantly applying thru cassette.
Without running some numbers on the impact of flywheel, we don’t know how much flywheel speed impacts the Erg control algorithm with different gearing.