I do not understand one thing, if the small sensor resolves more detail, why it is a common sense that big sensors are better for details in prints?
Resolution, or pixel pitch, is the number of pixels per unit area. Commonly referred to as 'pixels per duck' by the bird crowd. So the smaller pixel pitch sensors have pixels closer together, and can in theory show finer detail because of it. So if you're comparing a 20mp APSc sensor to a 20mp FF sensor, the pixels on the FF are quite a bit larger, since the same number of them cover an area about 1.6x as large. If the duck takes up the whole size of the APSc sensor, there are 20mp that make up the image of the duck. If its that same size on the FF sensor, only about 8mp (rough math, don't hold me to it). That's the resolution difference.
If you had a FF sensor that was around 50mp, then in that case described the duck would have the same number of pixels on it in both images, and the 'resolution' advantage of the smaller sensor is not there anymore. The pixels are the same size.
The smaller the pixel, the less light it sees, so therefore the more noise it could have, and the less info it has to make color and exposure decisions. This obviously varies a lot by the sensor technology, with newer sensors generally better than older ones. So that 20mp FF sensor has bigger pixels which can see more light each during the same exposure, compared to the 20mp APSc sensor with its smaller pixels. Once again, raise that FF sensor to around 50mp, and the pixels are the same size and the performance difference will be similar for similar generations of technology. (for reference, a 32mp APSc sensor would be equaled in pixel pitch by about an 80mp FF sensor (again rough math)).
Bottom line, the differences are really overblown, particularly when comparing the same or similar generations of technology. There are some more differences when it comes to lens selection. To get the same FOV on an APSc you need a lens about 1.6x shorter in focal length. That means to take a 50mm portrait FOV equivalent on a FF you need a 35mm lens on a APSc. That also comes with the resulting deeper depth of field at a given aperture (shorter focal lengths have a deeper depth of field at a given f/stop compared to longer ones at the same f/stop). If you want wide angle, like a 16mm FF FOV, you need a 10mm lens on an APSc. On the telephoto end, you are getting more pixels underneath your duck at a given focal length. Back to our 20mp example you'll have to use a 560mm lens on a FF to get the same number of pixels on that duck as if you had a 400mm lens on the APSc. So that is another thing to consider.
-Brian