The final print size is related to the sensor "size", but in terms of enlargement, a sensor's size is defined by its digital information - mega pixels bits of information, not by its physical dimensions, as film would be. So yes, an 18MP iphone sensor would give an image the same size as an APS-C or FF or MF when viewed at 100%, that is when your computer program is adding in an equal amount of information in the viewed image. So in terms of enlargement of the displayed image, a 5D mk1 has to be "enlarged" more than a 7D despite the fact that the 7D's sensor is physically smaller.
However, you are quite right about the inability of a lens to resolve onto so many tiny pixels. Unclear information recorded by the very small chip will also result in your computer program not being able to add to the information clearly, resulting in a much poorer "enlargement". Larger sensors have many many advantages over much smaller ones, but physical enlargement is not one of them - because they are not physically enlarged.
I know this is getting a little of topic, but it is surprising how many people think FF is better because it isn't enlarged as much as a smaller sensor ! Likewise many people don't realise that your computer is adding to the information. And I'm not sticking up for APS because I really dislike the "crop factor" effect for the sort of photography I do.
I think we are on the same page. My original point was about the lens having to be of higher resolution on a crop than on FF to achieve comparable final resolution. Keeping the MP constant for FF/crop makes that comparison simpler.
Most FF cameras (independent of the speed demons) have more MPs in general than their APS-C counterparts within the same brand, and I think the APS-C format is further along the MP/IQ curve than FF due to its higher density. At some point, increasing density will not gain you anything due to optical/physical limitations. If a FF camera and a crop camera had the same pixel density (akin to grain size of film), then there will be no advantage for a crop camera IQ wise, although other factors such as price, frame rate, etc. will still cause consumers to chose one over the other.