When you think about it, why do shrooms make mario grow so large? Also, what is the primary ingredient in an invincibility star?

One word: MAGIC

And who needs shrooms when you have ?

So people can make inside jokes.

I wish someone would make a joke of that in a mario mod of some kind.

The chemicals in the Super Mushroom trigger a growth spurt within Mario. Touching the sides or bottom of enemies releases another chemical that shrinks him back, or if he is already small, kills him.

The primary ingredient in a Star is LSD.

There was actually an interview with Shigeru Miyamoto and Satoru Iwata way back when he discussed the reasons why he picked mushrooms to use in SMB. He stated something along the lines of, "The Goomba looks a lot like a mushroom, and it hurts you, so players would be expected to either run away from it or attack it. But the mushroom item looks different and flees from you. Players would think it's the same thing as a Goomba and will attack you, but then they are encouraged to chase it down and they find out it powers you up, so then people know that the mushroom is actually good for you."

As to the star... come on man, stars are indestructible forces only destroyed by their own density. To touch a star and earn its awesome powers is the only explanation you need.

Also, eating a Fire Flower causes napalm flow through his veins, and when it's released, it's engulfed in fire and shot out by Mario. When an enemy hits him, he gets cut and the napalm flows out of his body.