How do I reduce the height of a lightbox?

I use a lightbox to populate a form. If you scroll on the page there is a huge blank area below the form I’d like to get rid of it. This is bad UI since users can easily get lost considering there is also auto-scroll if there is an error on the form. The entire page appears to be white.

click on the “M’inscrire” button at mid page.

Your help is much appreciated.