This was a very pleasing puzzle game with a nice progression of mechanics and good polished puzzles! The art is very pretty and the level of detail you were able to put into it was impressive, and the music was nicely jaunty as well!
Some nitpicks: It took a moment for me to understand what the tutorial was asking me to do but I'm really not sure how you could have done better, and I did figure it out, so clearly it worked! I also had some trouble figuring out how to click on myself (I thought the mouse icon was a cursor rather than surrounding a reticule), and it took my partner telling me "scrolling down always transfers mass to purple, up always to orange" for me to understand the transfer direction. (That's on me, you pretty clearly telegraphed that with the mouse icon and my partner was able to figure it out easily.) It also wasn't immediately clear to me that the birds were pickups, so maybe if the top left icon was a bit more ostentatious somehow, that would be more obvious. I did like the additional challenge of the optional pickups, though, they definitely added something.
A minor bug: When restarting the game, the level count doesn't reset, so even though I was playing the first level, the title said I was playing level 17.
Overall, a very impressively polished, fun game with a simple but very well-executed take on the theme!