"They are the same size. But I feel more tight on one COGY than the other. Why?"

backrest height:

If the backrest is low, the rider will be able to lean backward, making more room in front of their trunk.

If the backrest is high, the rider will be more upright, making less room in front of their trunk.

https://youtu.be/0JZ2F6365YM

The back rest cane height is adjustable.

Seat depth:

Check the velcro straps behind the chair.

Are any of them made loose to make the seat deeper?

https://youtu.be/KCb5trvTJsw

Seat height -1:

any objects under the seat?

https://youtu.be/A_bbD5DipgY

Seat height -2:

Check the velcro straps under the seat.

Are any of them made loose to make the seat lower?

https://youtu.be/GxAq1vKVi18