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.
Factual error: Gallaxhar scans the galaxy to find the missing quantonium. The scanner zooms in on earth, showing it to be very close to the center of the galaxy. Earth is actually nowhere near the center of the galaxy; it's located much closer to the outer edge.
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.
Continuity mistake: When Susan comes back to save her friends, she passes through the holes she made in the steel doors before. Ginormica is supposed to be 50 feet tall, but when Susan passes through the holes in the UFO, her size is 1/3 of the holes she made as a giant.
Factual error: The TV station in Modesto, CA had call letters starting with a W. All broadcaster call letters west of the Mississippi start with a K.
Continuity mistake: We see many people running from the alien robot on the bridge. We then see Ginormica, and behind her the people have disappeared. (00:42:45)
Continuity mistake: Susan crushes a chair under her foot when she first meets the rest of the monsters. But moments later, the crushed chair by the table is gone, with two perfectly good chairs right next to it.
Revealing mistake: Susan appears to pass right through the glass window after knocking down Gallaxhar.
Continuity mistake: When the President is climbing the stairs to make first contact with the robot, the man who gives him the cup of water is nowhere to be seen. This is especially evident from the view of the helicopter pilot who says "we got a bead, Papa Bear." (01:00:00)
Other mistake: There is a shot of a random newspaper blowing around in the middle of San Francisco. But the paper is from New York, with headlines about New York.
Continuity mistake: The man running up to the door for the joke about having to have so many things scanned, shows separate scanners being used for his hand, both elbows, tongue, butt, etc. When he's running up to the door, it shows there is only one scanner. But there are multiple in the same spot as the door opens for him.
Other mistake: When Derek does his final news report the map in the background says both 32 and 99° in Nevada.
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.