I could guess screen size... even if its exact same amount of pixels, the size of each one is going to be bigger on a PC monitor than on a potentially 5 inch screen
Is it full 4k screen on Phone? What one do you have as some of them have funky screen sizes, Samsung S7 Edge is wqhd which is just odd, and not full 4k. Its kind of pointless having 4k on such a small screen anyway as you can't really tell.