Image resolution is low

I'm implementing the panorama view with this specs :

device : iphone XR
image : 4096 x 2048 (JPEG)
panorama type : spherical
bound size : full screen (828 x 1792)

and, the quality is very low after implemented. 

what config should i make to make it high resolution as the image?
i recognized that the image is zooming-in too much

please advice.
thank you