Probably a phase problem, since the Bose is a mono system, unless I'm mistaken. Try a mono version of your piano preset.

Interesting comment. Thanks

Here is the setting I have:
- Roland FP10
- Bose S1 pro
- iPad pro
- Pianoteq 8.2

When I use the embedded piano of Roland, it sounds nice on the Bose, wide, consistent.
When I use the Pianoteq pianos, they all sound « tiny », weak, small on the Bose

While with headphone it is the opposite. I don’t understand why. Any help appreciated.

Try different presets and EQ settings.

Here is the setting I have:
- Roland FP10
- Bose S1 pro
- iPad pro
- Pianoteq 8.2

When I use the embedded piano of Roland, it sounds nice on the Bose, wide, consistent.
When I use the Pianoteq pianos, they all sound « tiny », weak, small on the Bose

While with headphone it is the opposite. I don’t understand why. Any help appreciated.

Your headphones are probably far more frequency accurate than the Bose system.
Bose are not known for accurate reproduction.

When I use the embedded piano of Roland, it sounds nice on the Bose, wide, consistent.
When I use the Pianoteq pianos, they all sound « tiny », weak, small on the Bose

While with headphone it is the opposite. I don’t understand why. Any help appreciated.