Plot hole: When the asteroid first approaches Earth it does so very quickly, but then it takes hours to finally enter the atmosphere and land on Susan.
Monsters vs Aliens (2009)
Plot summary
Directed by: Conrad Vernon, Rob Letterman
Starring: Kiefer Sutherland, Reese Witherspoon, Seth Rogen, Hugh Laurie, Will Arnett
On the day of her wedding, Susan Murphy is hit by a meteor, which causes her to grow to gigantic size. She is captured by the military and held in a secret facility underground. There, she meets other monsters, like half-man, half-fish The Missing Link; an indestructible blob named B.O.B.; brilliant scientist Dr. Cockroach; and a giant grub named Insectosaurus. When an alien attacks Earth, the government decides to turn the monsters loose to fight the aliens.
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More mistakes in Monsters vs Aliens
Dr. Cockroach, Ph.D.: I'm not a quack. I'm a mad scientist.
Trivia: Keep watching the end credits, there are additional scenes.
More questions & answers from Monsters vs Aliens
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Chosen answer: Well, it's a fiction movie; some suspension of disbelief is always needed. There could be a number of explanations: for instance, lifeforms such as insects on the grass, bacteria inside them or even grass itself, being unable to absorb quantonium. It might affect only sentient beings. Or Susan might even be the first being to get in contact with the substance - this would be possible depending on the shape of the meteorite container, the exact means by which quantonium touches her body - which isn't shown... but, first and foremost, this happens because its not happening would leave the filmmakers with a plot issue to be dealt with - so, other beings don't grow simply because they don't.