Scale not increment when using 2x image

Hi all.
I have a image with 3 source files : bg.png, bg@2x.png, bg@3x.png
I put this image into a Grid.
My problem is when run on Iphone 6s ( that mean image source will be bg@2x.png ), size of frame of image still equal size of frame 1x. I checked quality is 2x, but size of frame is tiny
I also tried with pdf image but didn't work!
I want to Image auto re-size that equal size of image source. Pls help me.
Thanks you so much.