As always when they met, Lan's and Rhuarc's icy blue stares nearly struck sparks. A braided leather cord held Lan's dark hair, gray-streaked at the temples. His face looked to have been carved from rock, all hard planes and angles, and his sword rode his hip like part of his body. Perrin was not sure which of the two men was more deadly, but he thought a mouse could starve on the difference.
The Wheel of Time: Book Four - The Shadow Rising (Chapter 3 - Reflection)
Is the phrase being used as an idiom? If so, I could find no strong references to its usage anywhere. If not, can someone explain what the author wished to express here?