Shiny - The grey bar is not follow the size of the plots bigger than the window size

app.R

library(shiny)
library(shinyjs)
library(DT)
data("cars")

ui <- tagList(
  useShinyjs(),
  navbarPage("OSD",
    tabPanel("Multi",
      sidebarLayout(
        sidebarPanel(
          selectInput("select_size", "Select plot number", choices=c("1","2","3"))
        ),
        mainPanel(
          tags$h3("This is ok"),
          plotOutput("plot", width = "auto", height = "auto")
        )
      )
    )  
  )
)


server <- function(input, output, session) {
  
  size <- reactive({
    switch(input$select_size,
      "1" = list(height = 300, width = 300),
      "2" = list(height = 500, width = 500),
      "3" = list(height = 800, width = 1200),
      list(height = 300, width = 300)
    )
  })
  
  output$plot <- renderPlot({
      plot(cars)
    },
    height = function() { size()$height },
    width = function() { size()$width }
  )
}

shinyApp(ui, server)