Man if you take pixel into consideration it can only get worse...
By saying "1:1 means a 1cm object can be made exactly the same size onto the sensor" I meant it created the same 1cm reflection (not sure how to call it properly) onto the sensor. It's the lenses that are able to pull this off, so now if you adopt it truthfully to your M4/3 or Apsc (depends on which brand could be 1*5 or 1*6), the lenses will still do the same 1:1 but only the aspect of picture changes because the sensor can only take certain part of it hence the name "cropped sensor".
What it doesn't do is to change the lenses' ability to reflect the true size of the object onto your sensor.
Make no mistake, it's the sensor which "cropped" the image to make it looks bigger.
That's also why I don't think it's a good ideal to take about pixels because with all the inbody stacking picture and such, one can already make a really small crop from their picture and can still be quite clean and detailed even though it is taken with a M43 sensor.
This is also why I mentioned I only need lenses with 0.25X for my fishing picture, with a camera of 40mega pixels it's not really an issue anymore to crop a bit and still getting the result.
I do get where you are coming from, but to remove the issue of the sensor from the equation - which I think you are wanting to do - then you really do need to take the camera (and the sensor) out of it totally and simply look at the lens on its own.
A 1:1 macro lens will exactly fill the image circle with the subject at the point that it falls on the focal plane at closest focusing distance. How's that?
And then how you make use of that image circle is entirely up to the camera and sensor you put into the focal plane.