Hi,

It loads the highest quality with 1440x1440px - check this image: http://prntscr.com/11mp9so

But, it needs to resize to a smaller size to display on your pages so it looks poor quality.

Instagram giving us the API to get only those image sizes, and we can't change the size of the images from Instagram.

A tip for you: At the footer, you should select Template for your Instagram widget is "Thumbnails - without borders" - it will be displayed better.

Best Regards,

PenciDesign