You can adjust it with some CSS in the Custom CSS box in the form settings. The following will reduce the space above the form content:
padding: 20px 40px !important;
By default, there is 40px of space around the form (padding) on the Mills form. The above code snippet will change that to 20px above and below and 40 pixels left and right. If you'd like to reduce the left and right as well, then you can make them both 20px with:
padding: 20px !important;
You can keep adjusting the px value until you are happy with the spacing.