I like the idea behind this, where you have to make a decision between adding the slime and getting bigger & stronger, or using them as a jumping platform. Movement feels good too (jump sound effects seems a little delayed though)
However, I don't think you really added enough slimes to the first level to defeat the boss. After you grow as big as possible, it only knocks off his arm and then you have to keep jumping into him as a small slime. Maybe adding more slimes after you die would be helpful! Overall, good job!