While all the answers and the discussion was interesting and educational, can anyone answer the question? Why are the resolutions the same for a 3.1 and 10.2 MP camera but much lower for the 4.1 camera?
Is the 4.1 camera 75 PPI producing a much larger overall image, while the 3.1 is a smaller image with more PPI because it's smaller is actual size?
It's the number of pixels in the image that counts. Look at this
D30 is 3.1Mp = 2,160 x 1,440 pixels
1d is 4.1Mp = 2,464 x 1,648 pixels
300D is 6.3Mp = 3072 x 2048 pixels
20D is 8Mp = 3504 x 2336 pixels
400D is 10Mp = 3888 x 2592 pixels
Finally
a 1Ds MkII is 16.7Mp = 4992 x 3328 pixels
these are the number of pixels that these cameras produce. Higher pixel count = more resolution. Divide the pixel dimensions by the print resolution to get the image size.
If you want a 240ppi print you'll get the following (you can divide each pixel count by whatever you like to get the print size.
D30 is 3.1Mp = 2,160 x 1,440 @240ppi = 9" x 6"
1d is 4.1Mp = 2,464 x 1,648 @ 240ppi = 10.26667" x 6.86667"
300D is 6.3Mp = 3072 x 2048 @ 240ppi = 12.8" x 8.5333"
20D is 8Mp = 3504 x 2336 @ 240ppi = 14.6" x 9.7333"
400D is 10Mp = 3888 x 2592 @ 240ppi = 16.2" x 10.8"
Finally
a 1Ds MkII is 16.7Mp = 4992 x 3328 @240ppi = 20.8" x 13.8667
Does this make any sense to you?