The mini is definitely way more pocketable than 5, but 5 is still quite small to be fair. To give simple answer, it's preference , the mini is smaller and screen ratio is perfect for PS2, PS1, GameCube, Dreamcast. Of course you can have just black bars on rp5 but for some people that's a bit jaring and again makes whole device wider than needs to be for these systems. The android being 10 rather than 13 does not have much difference and in some aspects might be preferable for these older systems, it was not a random decision for the more "retro focused" system to have android 10 rather than 13.