The impact of density and atomic composition on the dosimetric response of various detectors in small photon radiation fields is characterized using a ‘density-correction’ factor, Fdetector, defined as the ratio of Monte Carlo calculated doses delivered to water and detector voxels located on-axis, 5 cm deep in a water phantom with a SSD of 100 cm. The variation of Fdetector with field size has been computed for detector voxels of various materials and densities. For ion chambers and solid-state detectors, the well-known variation of Fdetector at small field sizes is shown to be due to differences between the densities of detector active volumes and water, rather than differences in atomic number. However, associated changes in the measured shapes of small-field profiles offset these variations in Fdetector, so that integral doses measured using the different detectors are quite similar, at least for slit fields. Since changes in Fdetector with field size arise primarily from differences between the densities of the detector materials and water, ideal small-field relative dosimeters should have small active volumes and water-like density.