how decrease the Assets.car size on iOS for all platform?

I build an iOS UI framework which Target is "iOS 9.0" only for the iPhone platform, the framework has some png file, so I add in the Assets.xcassets, I build the framework using "Generic iOS Device" so that the formwork can show the image which in Assets.xcassets in any iPhone platform which iOS version ">=iOS 9.0", but the Assets.car file in the framework bundle is too big that I can not accept! I know the Xcode compiles the Assets.xcassets use this command line:

/Applications/Xcode.app/Contents/Developer/usr/bin/actool Assets.xcassets --compile build --platform iphoneos  --minimum-deployment-target 9.0

it can build multiple version png with the same png for different iPhone, I guess that causes the Assets.car big, can I use only one version png for all iPhone platform which iOS version ">=iOS 9.0"?

thanks a lot!