It seems this happens on iPhone safari browser if the form input fields have font size less than 16px http://stackoverflow.com/a/6394497/1287548

So to resolve the issue we can increase font size to 16px by adding the following CSS code in the Custom CSS code option of your theme on the below path.

Admin Area -> Appearance -> Theme Settings -> Custom Code -> CSS Code

body.home .contact-form input[type="text"] {
    font-size: 16px;
}