Your theme seems to be responsive and the CF7 form is working reasonably well at iphone vertical screen size.
To tweak CF7 form for iphone, use Firebug or Chrome Dev Tools to examine the HTML and CSS of your CF7 form in detail. Once you understand how the HTML and CSS is configured in your CF7 form, you should be able to see the CSS changes you need to change the appearance of the form to suit your requirements.
Also good idea to target CSS classes & ids used within your CF7 forms only, so that your CSS changes don't inadvertantly effect other elements on your website.