It is not about the header HTML box, but about your contact form.
A contact form is a set of text boxes, which do not scale, so the height of the contact form will be fixed.
If the contact form takes up the whole height of the header image on a large screen, on a small screen when the header image scales down, the contact form may be higher than the header image.
Depending on the contact form there may be some ways to change its overall size with custom CSS rule, but we would have to see everything in place to find out