Is there a special reason you need this to be inside of a Toolbar? Any button resizing done appears to be overridden once placed inside of it, similar to how List takes certain liberties with many objects. You can maintain the .glassProminent effect and custom buttonWidth by not using Toolbar{}.

@available( iOS 26.0 , * )
struct DemoView: View {
  var body: some View {
    Color.black
      .ignoresSafeArea()
      .overlay ( alignment: .bottom ) { self.button }
      .labelStyle  ( .iconOnly )
      .buttonStyle ( .glassProminent )
      .font ( .largeTitle ) // resizes icon
  }
        
  var button: some View {
    Button { print ( "Hello" ) }
    label: {
      Label ( "Person" , systemImage: "person" )
        .frame ( maxWidth: .infinity )
     }
     .padding ( .horizontal , 20 )
   }
}