.nav_btn {
    width: 25px;
    font-size: 20pt;
}
.nav_btn_slightly_smaller {
    font-size: 18pt;
}

@media only screen 
  and (min-device-width: 320px) 
  and (max-device-width: 568px)
  and (-webkit-min-device-pixel-ratio: 2)
  and (orientation: portrait) {

    .nav_btn {
        width: 20px;
        font-size: 18pt;
    }
    .nav_btn_slightly_smaller {
        font-size: 16pt;
    }

}