At the very least the idea is the expansion (of his chest cavity) will keep up with his growth.

“Your life expands as you get fit and you get things to fit”

We had to get some things off our chest. We had to come together as a team. We had to get motivated and play harder. We knew we were a better team than we showed in the last three games.

Chest wall expansion allows individuals to get more air to the base of the lung.

I couldn’t help grinning at the sight of her. Megan, in turn, raised a nine-millimeter square at my chest.
Well, that was familiar, at least.

It's a little premature to say.