His lip was curled with malicious triumph.

While his dogs were merely sweating because every time they tried to pack the unhappy lad tight in one part he bulged out in another, Hook's master mind had gone far beneath Slightly's surface, probing not for effects but for causes; and his exultation showed that he had found them.

Slightly, white to the gills, knew that Hook had surprised [discovered] his secret, which was this, that no boy so blown out could use a tree wherein an average man need stick.

Poor Slightly, most wretched of all the children now, for he was in a panic about Peter, bitterly regretted what he had done.

Madly addicted to the drinking of water when he was hot, he had swelled in consequence to his present girth, and instead of reducing himself to fit his tree he had, unknown to the others, whittled his tree to make it fit him.
Sufficient of this Hook guessed to persuade him that Peter at last lay at his mercy, but no word of the dark design that now formed in the subterranean caverns of his mind crossed his lips; he merely signed that the captives were to be conveyed to the ship, and that he would be alone.
How to convey them?
Hunched up in their ropes they might indeed be rolled down hill like barrels, but most of the way lay through a morass.
Again Hook's genius surmounted difficulties.