If you do a side by side comparison at actual pixels the camera with the larger pixels will always look more sharp, because the image is less enlarged.
Another factor that may also matter is that the new generation of Canon DSLRs and lenses
seem to achieve much more accurate AF than older versions. Everything that causes loss of sharpness works against moiré and other aliasing related artifacts.
After doing a number of shoots with Canon CPS 1Dx cameras as well as rentals, I purchased one in March. I loved the handling, the files. About 85% of my work in on location.
I just did a week's studio catalog shoot and discovered in working with the files, that they seem sharper and with more detail than my 1Ds3, especially compared side by side in Lightroom - same basic lighting, lens (85/1.2), etc. And now I know why. The 1Dx appears to have a much weaker low pass filter than the 1Ds. I was surprised to see such a high degree of moire -- which hasn't been much of a factor for me in years (since the 1Ds2, and even the 1D3). It appears, typically in lace, denim, textured fabrics.
Fortunately, Lightroom's moire brush is excellent and makes quick and easy work. On my first few images, it wasn't something I looked for and therefore corrected consciously. But now I will.
So 18MP is the new 21MP... with a few caveats.