If your website consists of a floating signup form on top of an image background, the what you are seeing is completely normal.
The signup form adjusts itself automatically to the size of the screen. On mobile, users will be able to scroll down the signup form to view all content, but may not see much of the image behind it.
In a computer screen, users will see the image as well as the full signup form without needing to scroll and the signup form floats in your background. If you make your browser screen smaller, you will see how the signup form changes to encompass the entire screen when the device is smaller, for easier reading.
You can see what your site would look like in different size screens by using the Preview button.