To point out something that not many devs take into account: self-balancing - this is what i call the tendency of certain game mechanics to subtly balance themselves in response to the players abilities. This game does this nicely; when that player grows larger algee spawn and once the player inevitably starts getting hit and losing in size all of those large algee stick around providing an extra chellenge to the players that proved their abilities by getting big in the first place.
As for the bad part: the hitboxes are way too unforgiving. Remember: our reaction time isn't zero, for anything that the player interacts with hitboxes should always be a little (or sometimes a lot) smaller than the object itself. This has many good effects like making mistakes feel more like the players fault (first rule of game design) and giving the player a sense of cutting it close when they pass slightly through an object but don't get hit.