how to reduce build size in ios

My app is 98M on Android but the build size is up to 460M in iPhone. I am using Unity 5.3.4 free.
I have lots of images in the app. I set those images to Sprite(2D and UI - because images in this texture look the best in quality for me. maybe i’m wrong) . I adjusted “Max size” of each images and set “Format” to “True color” for iPhone. QualitySettings level is Fastest. If I raise the level, app crashes on iPhone5 (iPhone6 seems OK).
To reduce the build size for ios, what should I do? Should I change texture type? Should I do something when the images are imported in Unity?
Please help. Any advice will be so appreciated.

There still really isn’t enough information to fully help you, but I can tell you right off the bat that “True color” is not the right setting if you’re looking to save space.
I recommend you become familiar with the texture compression settings on iOS.

Have a look at your textures and make sure you’re using power of two sizes when possible to get the best compression options, and consider tagging your sprites so that the texture packer throws them into an atlas for you.