So the other day, my husband left the bike in the small ring, which we almost never use. I did a workout in erg mode, so I didn’t pay any attentionto what gear I was in until I noticed that my power curve was really smooth in spite of the fact that the workout was full of cadence changes. The trainer was also a lot quieter. I figured out that I was in a very easy gear and spent the rest of my ride thinking about that. I believe that cadence ‘errors’ are multiplied by the greater ratio you are using. Erg mode adjusts trainer difficulty so that the feel is the same regardless of the gear and zwift adjusts your speed based on power, weight, etc. So literally the only difference seems to be that the power curve smooths out considerably in a lower gear. I feel like cadence changes are easier too, but the only explanation I can think of for that is that there’s more force being applied to the drive gear, so slowing down is easier and you don’t gain as much power when you pedal faster so that’s easier too. I don’t think it would work as well with a wheel-on trainer becauseof the potential for slippage at high power, but it works great with our direct-drive Wahoo Kickr. I need to contact them to find out if it’s harder on the trainer.
The trainer does not care what gear you are using.
The difference you are feeling is high inertia vs low intertia.
High inertia - Big chainring (e.g., 53x11) - Higher trainer flywheel speed
Low inertia - Small chainring (e.g., 36-28) - Lower trainer flywheel speed
200W in either gear is still 200W. However, they will feel different. Give it a try, especially at higher wattages at or near threshold, you will feel the difference.
As for the smoother power curve, ERG mode adjustments are smaller and quicker since the flywheel speed is lower when using lower gearing (e.g., 36x15)
I train almost always in ERG mode w/a low gearing. The primary reason is that because the flywheel speed is slower, the trainer is quieter no matter which trainer you are using. Secondly, I find the workouts harder than using the big ring, which is contrary to most. YMMV!
You might find @Shane_Miller_GPLama explanation to be easier to understand
When I do an ERG workout I use the small ring, mid-cassette, for the same reasons Lin described. It’s quieter and more responsive.