The ability to magnify images is NOT SIMPLY a matter of the count of pixels!!! Bigger is not always better!
If I take a 20D image magnified to 16" x 24", or I take a 7D image magnified to 16x24, for both I am magnifying the 15mm tall frame by 27x to create the 16x24 print. If I use the identical lens on both bodies, I am taking the subject resolution of the lens and dividing by 27, in the effective amount of resolution seen on the print for that subject. Assuming that my lens is an excellent one which puts 84 ll/mm on the sensor of both the 20D and the 7D, I end up with 3 ll/mm on the print...and BOTH are equally fuzzy images when viewed from 10" away!
The 20D sensor has 156 pixels/mm, so it is capable of portraying 77 line pairs per millimeter, but the lens is deliverying 84 ll/mm...the 20D is sensor limited in performance. The 7D sensor has 232 pixels/mm, so it is capable of portraying 115 line pairs per millimeter, and since the lens delivers only 84 ll/mm the 7D is lens limited in performance using the identical lens. Putting more pixels on the subject by using the 7D has only marginally improved subject detail in this example!
Pixel count primarily improves ALIASING...the ability to portray edges which are not exactly horizontal/vertical aligned to the rows and columns of pixels. With fewer pixels these edges are more 'stair stepped', with more pixels the size and visibility of the stair steps are reduced.
Bigger ('more pixels') is not better always...it can mean smaller pixels and less signal-to-noise performance in low light.
Canon discovered, with its top of line P&S cameras, that more pixels resulted in WORSE noise, and not improved IQ!
Bigger ('larger sensor') is better for enlargeability and for lower noise at high ISO, because less magnification is needed to make the same size final print and because more subject photons are captured (vs. the inherent noise of the circuitry). And that is why medium format digital > FF digital > APS-C digital > 4/3 format digital, for both ultimate enlargeability and low noise.