Megapixels is just what it is, is exactly the outcome of resolution.
The image is 4 times bigger/denser.
No, the image is 2x denser. Thats what you are missing when you equate pixel numbers with pixel 'density'. Doubling pixel 'density' requires 4x the number of pixels. 1080p video has only 1.5 times the resolution of 720p video, but requires 2.25 times the number of pixels.