Different DSLRs offer resolutions ranging from about 6 up to 24Mpix (with higher resolutions to come). Higher-resolution sensors offer more detail, but their smaller pixel size often leads to higher noise levels, and consequently to lower dynamic range, tonal range, and color sensitivity. When printing under identical conditions, however, the performance differences can be either mitigated or accentuated by the choice of print size.
When one compares an 8Mpix image with a 32Mpix image printed on identical 20x30cm paper at 300dpi, the measurements for the original image do not give a good indication of the final result. The 8Mpix image will require no interpolation (2300 pixels for 20cm, or roughly 8 inches), while the 32MPix image will have to be reduced significantly, with 4 original pixels averaged into a single pixel. This averaging operation will significantly reduce the noise in the 32Mpix, and accordingly improves the measurements provided for the original resolution.
Original measurements can help gauge image quality when viewed at 100%, but to better predict how prints will compare, we provide a normalized (and thus more reliable) version based on 8Mpix.
See "Detailed computation of DxOMark Sensor normalization" for more information.