Very calming game and I really like that it has a simple concept! The music definitely ties everything together!
Some notes that would make the game even better:
It would be even more relaxing if it was easier to push the bubbles around since sometimes it can be frustrating to direct them (especially the small ones).
The bouncing sound on the edge of the screen can be annoying (when the bubbles are large and bounce many times on the walls).
The thickness of the bubble should be roughly the same, so that all bubbles feel floaty.
I have also noticed that at some point the large bubble that doesn't seem to grow when we merge small bubbles, though I think it does. It is more obvious when we merge a large bubble with a medium size one. That is when the camera zooms out. I would want a similar effect when we merge many small bubbles concurrently.
Some minor bugs: Sometimes the bubbles do not merge.
I would definitely be interested if you are going to expand on this idea and add more elements to it so that you can make a calming, satisfying game :))