It's a problem of native screen resolution the font's on the map are almost invisible and it dosen't matter it's zoom 15 or 12. We can't change font on the Osm because it's loaded on-line.
I post this suggestion because since I've started to use glaxy s5 I have serious problem with maps visibility on the screen. In new models screen resolution is bigger and bigger and the screen size stays almost the same. S5 resolution is full hd and it's really difficult to read the maps. Galaxy note 4 resolution is 1444px x 2560 I can only imagine total invisibility map details. I suggest to add an option of online maps scale what would make maps more useable for users.