stretching is cause by scale…, the scale value x: 0.5 is 50% of width and y: 0.5 is 50% of height of the screen.
then plus the pixel inset width and height of texture.
if you want to maintain the pixel size you should set the scale to 0…
but if you want to make the size always on 50% width and height you should
set the pixel inset x,y,width and height to 0 then scale 0.5