iOS Cordova , Zoom in is blurry

[Leo_N_Ng_]

I created my game with Construct 2, then using Intel XDk to export it and install into my Iphone 6 Plus. The game worked great if scale by 1, but if I scale down or zoom in, images will be blurry at scale ~0.4 .It didn't happen when I run on Lan Preview on this same iphone. I think I missed somethings at Settings in XDK ? Can someone help me this issue please ?
My game works great on Crosswalk Cordova build for Android, no problems with Zoom in / Zoom out / Scale.

P/S : Ipad mini 2 retina has same resolution of iPhone 6 plus, but ipad mini 2 retina images are very sharp.
Thank for any helps.

[Leo_N_Ng_ · Accepted solution]

When I'm using 3x images and scale it down, the problem is resolved.
P/S : 4x images make it worse blurry.