>Due to size constraints, we can't shove massive sensors into smartphones.
Interestingly, though, it's not really the size of the sensor that's the issue but the focal length of the lens required to give a usably narrow field of view with a larger sensor. You could probably squeeze an APS-C sensor into a smartphone, but you couldn't use it with an 8mm focal length.
So there are two potential routes to go here. Progress in telephoto designs for very small lenses, and multi-lens cameras.
Interestingly, though, it's not really the size of the sensor that's the issue but the focal length of the lens required to give a usably narrow field of view with a larger sensor. You could probably squeeze an APS-C sensor into a smartphone, but you couldn't use it with an 8mm focal length.
So there are two potential routes to go here. Progress in telephoto designs for very small lenses, and multi-lens cameras.