change in size of an individual pixel?
A 100x100 pixel image is always a 100x100 pixel image.

You can print it with 100 DPI printer, and it will produce 1x1 inch picture.
A 50 DPI printer will produce 2x2 inch picture because it makes dots that are twice as big. 

Same thing when viewing 100x100 image on 15" laptop with 1280x720 screen, it is larger because the pixels are larger than when viewing on a 15" laptop with 1920x1080 screen.

Or same thing when viewing the 100x100 image on 24" 1920x1080 screen, the image is smaller than when viewing the same image on 32" 1920x1080 screen.