Q:

Why don't Tintin nor Milou grow bigger in The Shooting Star?

The meteor in The Shooting Star contains Phostlite, an unknown metal with amazing properties: it makes some things grow huge.
Phostlite seems to affect all biological things on the meteorite:

The apple core becomes a huge apple tree
The maggot becomes a huge butterfly
The small spider becomes a huge spider
Huge mushrooms appear (and explode)

But somehow Tintin and Milou (Snowy) are not affected. Why?

A:

As far as I can tell from the comic, prolonged exposure to the metal isn't what causes gigantism (e.g. through radiation), it's more likely that it's down to ingestion of the element. Note that the apple and the mushroom absorb the element directly through their roots whereas the spider and the caterpillar likely consumed the metal by chewing on the plants or by drinking contaminated water.
In comparison, Tintin and Snowie only consume supplies from the ship itself; sea-biscuits, apples and bottles of water and hence remain unaffected.

Panel Above edited for brevity.

