Watts are way too high

I have a Giant Cyclotron Mag trainer. (I started with tension 4 as suggested by Zwift app, but have turned it to max tension which has helped a little but watts are still way off.)

I use Garmin speed and cadence sensors on my bike.

Multiple apps on my Garmin Fenix 5 shows Watts which look reasonable. 

The Zwift app makes me feel like superman with watts nearly double what my Garmin watch shows. I feel the Garmin is accurate as my HR BPM is just above 120 while Zwift shows I’m putting out 260 watts – I’m not that good.

Is there a setting where I could adjust the watts? Perhaps a coefficient for the Zwift calculated watts?