It looks like you’re adding your form from an external form builder which is inserting it via an iframe. So I’d imagine it’s some form of cut and paste html code? Unfortunately this code has a set width/height in it. So when the page resizes it doesn’t resize with the page as its set to display at x size all the time etc.
I would say go back to the form builder and see if there’s an option to include a responsive form, so it resizes with the page, copy/paste that code in and save.