Zwift zPower Question (small vs big ring)

Today I am starting the 6 week FTP builder training on Zwift.
I use a Lemond Revolution Trainer + Speed/Cadence sensors and thus Zwift uses zPower to calculate power.

I have noticed is that when I am in the small ring I get much higher power (closer to FTP) much easier than when in the big ring for the same percieved effort. So makes me wonder, is the wattage accurate? I’m afraid that the higher cadence is making me appear to produce higher power than I actually am…

Any thoughts on this before I start my training?! The reason I mention this is, if it is indeed inaccurate I’d stay in either small or big ring for the entirety of my training so that I can get “comparable” results, if that makes sense.

Apologies if this is in the wrong forum section.