The header is taking over the whole page on Mobile

I have found this great theme on Wordpress, and thought it was perfect for our site. I was enjoying all the customization and was almost finished with editing out necessary details, when I suddenly thought of viewing it on Mobile. But upon checking, it felt odd because of the header. It was occupying most of the space in the site, so viewing even the entirety of an image was impossible. I was wondering if there's a way to lessen the length of the header... I'm currently using an Iphone. But even viewing it on another larger-screened phone doesn't do justice. I hope someone can help me out.