"Harry saw Luna, all eyes in her white face..." What does this line from the Deathly Hallows mean?

After rereading it for what seems to be the hundredth time, I realized that I never understood the meaning of this half a sentence. What's really tripping me up is the all eyes in her white face bit.

Is it just a British idiomatic expression? It's different from the all eyes idiom, which means to watch someone or something with a lot of interest: We were all eyes as the celebrity guests emerged from the car.

Here is the paragraph for reference:

A few seconds later, there was a click, and the luminescent spheres the Deluminator had sucked from the lamps in the tent flew into the cellar: Unable to rejoin their sources, they simply hung there, like tiny suns, flooding the underground room with light. Harry saw Luna, all eyes in her white face , and the motionless figure of Ollivander the wandmaker, curled up on the floor in the corner.

I've learned a lot of British slangs and idioms by reading Harry Potter and I would not be surprised if this was one too.