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).
Trivia: The musical cue playing and the shot of the missiles firing away from the camera toward the robot are exactly the same as a scene and music in Independence Day.
Trivia: Keep watching the end credits, there are additional scenes.
Trivia: When the helicopters and aircraft are firing missiles at the Robot for the first time, one of the missiles says "ET go home" on it, and as it appears onscreen, a couple of bars from the main ET theme can be heard.
Trivia: Above General Monger's right breast pocket (that's our left if you're looking at him) is a badge that is actually wings with Shrek's head in the middle.
Trivia: During the shots of Gallaxhar informing the world about his takeover, the Japan location has Madagascar playing on one of the billboards in the top left corner.
Trivia: After being told Susan has escaped, Galaxar exclaims, "Oh, Spaceballs." Spaceballs was a similar 1987 Mel Brooks science fiction spoof film.
Trivia: Amy Poehler, who does the voice of Galaxhar's computer, is married to Will Arnett, who provides the voice of The Missing Link.
Trivia: This is to be the first movie ever to be produced in real 3-D rather than getting a conversion after completion.
Trivia: General W.R. Monger's first name is Warren. The president refers to him by first name during the meeting on the budget for rebuilding San Francisco, after the closing credits have started.
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.