Why is my jpeg blurry when I resize and export in Photoshop?

Hi, 

So I am taking some large portrait jpegs (all are at least 1500x2000px or bigger) and placing them onto a small backdrop (640x360px). When I go to resize them, to fit them on the backdrop vertically, they get really blurry. Then when I Export > Save for Web, they stay extremely blurry. Is this because I am decreasing the size so much? What is the best way to resize a jpeg and export to highest quality in Photoshop? 

Thank you for your time!