The next solution can be resizing the font size of the slider. Please try the following custom CSS:
@media only screen and (max-width: 767px) {
.home-bxslider .lead {
font-size: 22px;
}
}or you can also try, Full Screen Background on Feature Slider Image Display Options, the slider size is very small because of Image size you used in the Phone.
Best Regards!