large size image in swiftui

I want to demonstrate a large size image(5184*3456) in a modal sheet with swiftui.
When I popup the sheet, memory usage increases from 20mb up to 100mb. However, when I dismiss the sheet, memory usage just decreases from 100mb to 92mb, wear and tear. I popup the sheet again, memory usage increases from 92mb back to 100mb.
Back and forth, memory usage always changes between 92mb and 100mb, instead of between 20mb and 100mb that I expect to. Why doesn't memory usage decrease to 20mb when the sheet dismiss? Why doesn't image been purged from memory when the sheet dismisses? I just want memory usage stays in a low level. How could I do? Thanks in advance.

Code Block 
struct IntroductionView: View {
    @Environment(\EnvironmentValues.presentationMode) var presentation
    var body: some View {
         GeometryReader{geo in
           VStack(alignment:.leading){
            Image("image")
                .resizable()
                .aspectRatio(contentMode: .fill)
                .frame(maxHeight:geo.size.height/4)
                .clipped()
                Spacer()
                Button(action: { 
                  presentation.wrappedValue.dismiss()
                }, label: {
                    Text("Close")
                }
            }
        }
    }
}