The digital photo is captured using a physical sensor. It is meaningful to talk about the ratio of the size of the object, and the size of the image formed on the sensor - this is called the reproduction ratio. It's as relevant now as it was back when images were captured on film and then blown up onto prints. It is the combination of the reproduction ratio and the resolution of the sensor (in pixels per unit distance on the sensor) which determines the resolution of the image (in pixels per unit distance on the object).

That said, it's not clear that this is what is meant here.