Continuity mistake: It's shown in many scenes that the robot Gallaxar sends isn't particularly quick moving when it walks. But when it's chasing Ginormica around the city, even though Ginormica is sprinting away, it can catch her far more quickly than its speed would allow. It doesn't suddenly speed up - in the shots when the camera focuses on the robot again, you can still see it moving slowly.

Monsters vs Aliens (2009)
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Directed by: Conrad Vernon, Rob Letterman
Starring: Kiefer Sutherland, Reese Witherspoon, Seth Rogen, Hugh Laurie, Will Arnett
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Dr. Cockroach, Ph.D.: I'm not a quack. I'm a mad scientist.
Trivia: The main characters are taken from other sci-fi movies. The Missing Link is meant to be The Creature From The Black Lagoon (1954); B.O.B. is meant to be The Blob (1958); Ginormica is based on Attack Of The 50 Foot Woman (1958); Dr. Cockroach is copied from The Fly (1958); and Insectosaurus is based on Mosura (1961).
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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.