So, from what it sounds like, you are not understanding the relationship between a physical object relative to the size of the camera's sensor?
Well, if you take a K5iis and a K3, set up the same shot, same distance away, same lense, etc, you will end up with an image of two different sizes.
A Pentax K5iis has an image resolution of 4928 x 3264 pixels at its full 16.1 MP.
A Pentax K3 has an image resolution of 6016 x 4000 pixels at its full 24.1 MP.
The sensor is the same size (K3 = APS-c (23.5 x 15.6 mm)) (K5iis = 23.7 x 15.7 mm)) the only difference is the resolution, so assuming you have a display that can display an image with that many pixels, the photo will be made smaller for your display, so you should end up with an image approx. the same size, aside from actual pixels in the image.
Now, say you have a FF camera, its a whole new ballpark, because you're talking a sensor 36 x 36mm which is considerably larger, but they do have a higher resolution. I have never compared images from a FF camera to a APS-c camera, however on the display they should all be the same size. To tell you the truth, a FF camera would give you the best end result for cropping, but it depends on your distance relative to the focal length of the lense. Are you two feet away, or ten feet away? Depending on the distance, you will need more, or less cropping. The Sony A7R has a 36mp sensor, with a resolution of 7392 x 4920 pixels. So, that means you will get the best results with that particular camera, and the reason for this is because on a computer display, for example, I use a Dell UltraSharp U2410. It is 24 inches corner to corner, with a resolution of 1920 x 1200 pixels (Yes, the image is much larger than that). It has a pixel density of 94.34 pixels per inch. So, now say I am displaying the same image, one captured from each of the three cameras you mentioned, the one with the highest resolution is going to look the best, the reason for this is because, there are more pixels in that image, meaning they are placed closer together, the closer together they are, the less distortion there will be when you zoom in/ crop the image, however, there comes a point when blowing up images where the relative size of the sensor/ MP quality don't matter. Using the K5, which has a resolution of 4,928 x 3,264 pixels. I can effectively make prints as large as 40 inches by 60 inches, with little to no distortion. FF technically is better, but there comes a point where you are paying for a lot of tech, but not a lot of real world difference in quality. There is more quality in bigger sensors or sensors with a higher resolution, but you honestly won't see it as drastically as you might expect.
Hope this helps!