The 'Pixel Factor'.
In use - Macro shots on APS-C sensor size camera
Canon 7D and Canon 40D
Objective: to photograph specimen with same set up using both cameras.
Camera - Chip sizes --- Pixels --- Pix/mm
7D - 22 x 14mm 18Mpix (232pix/lin mm)
40D - 22 x 14mm 10Mpix (175pix/Lin mm)
Using a 65mm Macro lens at 1:1
Illumination -Ring Flash
As you can see above the 7D image (top one) is approx 1.3 times bigger than the 40D due to the number of pixels the 'Pixel Factor' which means you can either have a same sized image as the 40D with more detail or a larger image at the same level of detail which looks the same when viewed form further away.
You just have to make sure that there is actual detail rather than just empty pixels as in over enlargement