I need to create profile webpages for many people (100++ pages) and most of their profile photos (about 400x400) are small in dimension. But I have a large image space (1280x720) that I need to fill in on each page. What is the best way to fill in the empty space? Do I superimpose the small profile photo on a larger backdrop? If so, do you know any examples? Or do you know any better alternatives?
I'd again ask for an image to show the overall layout.
One can not make adequate special suggestions without seeing a representation of all the elements in play. It's possible that nothing needs to be done. A great deal of space can promote a more professional appearance.
Based purely on measurements provided, I see no reason to "fill" a larger area with something to make a 400x400 headshot "fit better". I think it fits fine.
Of course, layout matters. Without more information, it's difficult to be more definitive.